b
$\begin{array}{*{20}{c}}
{\,\,\,\,\,\,\,C{H_3}\,\,\,\,\,C{H_3}} \\
{|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,|} \\
{{H_3}C - C\,\,\, + \,\,C - C{H_3}} \\
{||\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,||} \\
{O\,\,\,\,\,\,\,\,\,\,\,\,O}
\end{array}$ $\xrightarrow{{Mg - Hg\,/{H_2}O}}$ $\begin{gathered}
\begin{array}{*{20}{c}}
{\,\,\,\,\,\,\,C{H_3}\,\,\,\,\,C{H_3}} \\
{|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,|} \\
{{H_3}C - C\,\,\, - \,\,C - C{H_3}} \\
{|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,|} \\
{OH\,\,\,\,\,OH}
\end{array} \hfill \\
\,\,\,\,\,\,\,\,(pinacol) \hfill \\
\end{gathered} $