table and box over certain cells

Question

I was trying a way to make a box for a certain part of my table like the following

Is there a way to create the red box?

My code so far for the table:
\begin{tabular}{cccc}
    \toprule
          & $x$     & $y$     &  \\
    \midrule
    Β     & 10    & 90    &  \\
    \textcolor{blue}{B'}      & \textcolor{blue}{12}    & \textcolor{blue}{y=?}   & 2 \\
    Γ     & 20    & 70    &  \\
    \bottomrule
    \end{tabular}%

Answer 1

A simple solution with pstricks: I define a pair of empty nodes at two opposite corners and join them with a \psframe command:

    \documentclass[x11names]{article}
    \usepackage[utf8]{inputenc}
    \usepackage[T1]{fontenc}
    \usepackage{lmodern}
    \usepackage{booktabs}
    \usepackage{pst-node}
    \usepackage{auto-pst-pdf}

    \begin{document}
    \begin{postscript}
    \begin{tabular}{cccc}
    \toprule
          & $x$ & $y$\pnode[1.5em,1.8ex]{n2} & \\
    \cmidrule[\lightrulewidth](lr){1-4}
   B & 10 & 90 & \\
    \pnode[-1ex, -0.8ex]{n1}\textcolor{blue}{B'} & \textcolor{blue}{12} &     \textcolor{blue}{y = ?} & 2 \\
    Γ & 20 & 70 & \\
    \bottomrule
    \end{tabular}%
    \psframe[linecolor=IndianRed3, linejoin=1](n1)(n2)
    \end{postscript}

    \end{document}