\documentclass[a4paper]{book}
\usepackage{enumitem}
\setlist{nosep}
\usepackage[standard]{ntheorem}
\theoremstyle{margin}
\theorembodyfont{\normalfont}
\newtheorem{MyTheorem}{\hspace*{-0.5em}}[chapter]
\newenvironment{SampleText}{\begin{flushright}\vspace*{-.8\baselineskip}\bfseries}{\end{flushright}\vspace*{-.8\baselineskip}}
\newlist{MyEnumerate}{enumerate}{1}
\setlist[MyEnumerate]{labelwidth={1.7ex}, font=\bfseries, align=left, leftmargin={2.7ex}}
\begin{document}
\begin{MyTheorem}
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio. Praesent libero. Sed cursus ante dapibus diam. Sed nisi. Nulla quis sem at nibh elementum imperdiet.
\begin{MyEnumerate}
\item [A] Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio. \begin{SampleText}Sample text 1\end{SampleText}
\item [B] Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio.
\begin{SampleText}Sample text 2\end{SampleText}
\item [C] Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio.
\begin{SampleText}Sample text 3\end{SampleText}
\end{MyEnumerate}
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio. Praesent libero. Sed cursus ante dapibus diam. Sed nisi. Nulla quis sem at nibh elementum imperdiet.
\end{MyTheorem}
\begin{MyTheorem}
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio. Praesent libero. Sed cursus ante dapibus diam. Sed nisi. Nulla quis sem at nibh elementum imperdiet.
\begin{MyEnumerate}
\item [A] Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio.
\begin{SampleText}Sample text 4\end{SampleText}
\item [B] Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio. Praesent libero. Sed cursus ante dapibus diam. Sed nisi.
\begin{enumerate}[label=\textbf{(\roman*)}]
\item Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio. Praesent libero. Sed cursus ante dapibus diam. Sed nisi.
\begin{SampleText}Sample text 5\end{SampleText}
\item Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio. Praesent libero. Sed cursus ante dapibus diam. Sed nisi.
\begin{SampleText}Sample text 6\end{SampleText}
\end{enumerate}
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio. Praesent libero. Sed cursus ante dapibus diam. Sed nisi. Nulla quis sem at nibh elementum imperdiet.
\end{MyEnumerate}
\end{MyTheorem}
\end{document}
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\documentclass[a4paper]{book}
\usepackage{showframe}
\usepackage{xcolor}
\usepackage{enumitem}
\setlist{nosep}
\usepackage[standard]{ntheorem}
\theoremstyle{margin}
\theorembodyfont{\normalfont}
\newtheorem{MyTheorem}{\hspace*{-0.5em}}[chapter]
\newenvironment{SampleText}{\par\raggedleft\bfseries\color{blue}}{\par}
\newlist{MyEnumerate}{enumerate}{1}
\setlist[MyEnumerate]{labelwidth={1.7ex}, font=\bfseries, align=left, leftmargin={2.7ex}}
\begin{document}
\begin{MyTheorem}
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio. Praesent libero. Sed cursus ante dapibus diam. Sed nisi. Nulla quis sem at nibh elementum imperdiet.
\begin{MyEnumerate}
\item [A] Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio. \begin{SampleText}Sample text 1\end{SampleText}
\item [B] Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio.
\begin{SampleText}Sample text 2\end{SampleText}
\item [C] Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio.
\begin{SampleText}Sample text 3\end{SampleText}
\end{MyEnumerate}
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio. Praesent libero. Sed cursus ante dapibus diam. Sed nisi. Nulla quis sem at nibh elementum imperdiet.
\end{MyTheorem}
\begin{MyTheorem}
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio. Praesent libero. Sed cursus ante dapibus diam. Sed nisi. Nulla quis sem at nibh elementum imperdiet.
\begin{MyEnumerate}
\item [A] Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio.
\begin{SampleText}Sample text 4\end{SampleText}
\item [B] Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio. Praesent libero. Sed cursus ante dapibus diam. Sed nisi.
\begin{enumerate}[label=\textbf{(\roman*)}]
\item Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio. Praesent libero. Sed cursus ante dapibus diam. Sed nisi.
\begin{SampleText}Sample text 5\end{SampleText}
\item Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio. Praesent libero. Sed cursus ante dapibus diam. Sed nisi.
\begin{SampleText}Sample text 6\end{SampleText}
\end{enumerate}
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer nec odio. Praesent libero. Sed cursus ante dapibus diam. Sed nisi. Nulla quis sem at nibh elementum imperdiet.
\end{MyEnumerate}
\end{MyTheorem}
\end{document}
The color was just added for my visual reference.
The smart way: Calm down and take a deep breath, read posts and provided links attentively, try to understand and ask if necessary.
Now that I'm looking my preamble, I see that the only part I'm using \vspace is here, with the Sample text.
After that, I'm using \vspace also in my code to put some space before and after some lines.
I am a totally beginner in LaTeX, that's why I'm trying to find solutions that makes my text look perfect and I don't pay attention so much if my code is the proper for my situation.
I plan to ask a LaTeX consultant after my effort, to fine-tune my template\code.
NEW: TikZ book now 40% off at Amazon.com for a short time.
And: Currently, Packt sells ebooks for $4.99 each if you buy 5 of their over 1000 ebooks. If you choose only a single one, $9.99. How about combining 3 LaTeX books with Python, gnuplot, mathplotlib, Matlab, ChatGPT or other AI books? Epub and PDF. Bundle (3 books, add more for higher discount): https://packt.link/MDH5p