Explanation:
$\underset{\text{Oxalic}\,\,\text{acid}}{\mathop{\begin{matrix}
COOH \\
|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \\
COOH \\
\end{matrix}}}\,\,+\,4\,[H]$ $\underset{{{H}_{2}}S{{O}_{4}}}{\mathop{\xrightarrow{Zn}}}\,$ $\begin{matrix}
C{{H}_{2}}OH \\
|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \\
COOH\,\, \\
\end{matrix}+{{H}_{2}}O$