\usepackage{booktabs}          % preamble

\begin{tabular}{lccc} \toprule
  Case & Box A & Box B & Remarks \\ \midrule
  \multirow{2}{15mm}{A}
  & +X &     -1.0  & \\ \cmidrule(lr){2-4}
  & -X &     -7.9  & \\ \bottomrule
\end{tabular}