\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[ngerman]{babel}

\usepackage{xsim,lipsum,xcolor}

\xsimsetup{
  solution/print = true ,
  blank/filled-style = \underline{\textcolor{red}{#1}}
}

\begin{document}

\begin{exercise}[points=3]
  Erklären Sie den Begriff.

  \blank[width=4\linewidth,linespread=1.5]{}
\end{exercise}
\begin{solution}
  \blank[width=4\linewidth,linespread=1.5]{Die Lösung hier soll rot sein.}
\end{solution}

\end{document}
