% https://tex.stackexchange.com/q/305110
\documentclass[12pt, a4paper]{book}
\usepackage{xsim}

\xsimsetup{
  exercise/within = section ,
  exercise/the-counter = \thesection.\arabic{exercise}
}

\begin{document}
\part{EXCERCISES}
\chapter{Topic 1}

\section{Section}

\begin{exercise}
  I'm an excercise.
\end{exercise}
\begin{solution}
  I'm a solution.
\end{solution}

\section{Section}

\begin{exercise}
  I'm an excercise.
\end{exercise}
\begin{solution}
  I'm a solution.
\end{solution}

\chapter{Topic 2}

\section{Section}
\begin{exercise}
  I'm an excercise.
\end{exercise}
\begin{solution}
  I'm a solution.
\end{solution}

\section{Section}
\begin{exercise}
  I'm an excercise.
\end{exercise}
\begin{solution}
  I'm a solution.
\end{solution}

\setcounter{chapter}{0}
\part{SOLUTIONS}

\chapter{Topic 1}
\section{Section}
\printsolutions[chapter=1,section=1]

\section{Section}
\printsolutions[chapter=1,section=2]

\chapter{Topic 2}
\section{Section}
\printsolutions[chapter=2,section=1]

\section{Section}
\printsolutions[chapter=2,section=2]

\end{document}