a
Sol. $\quad M=N I A$
$N =1$
For $ABCD$
$\overrightarrow{ M }_{1}= abI \hat{ K }$
For $DEFA$
$\overrightarrow{ M }_{2}= abI \hat{ j }$
$\overrightarrow{ M }=\overrightarrow{ M }_{1}+\overrightarrow{ M }_{2}$
$=\operatorname{ab} I (\hat{ k }+\hat{j})$
$=\operatorname{ab} I \sqrt{2}\left(\frac{\hat{j}}{\sqrt{2}}+\frac{\hat{ k }}{\sqrt{2}}\right)$