b
Tension in the wire, $T = \left( {\frac{{2mM}}{{m + M}}} \right)g.$
$Stress = \frac{{Force/Tension}}{{Area}} = \frac{{2mM}}{{A\left( {m + M} \right)}}g$
$ = \frac{{2\left( {m \times 2m} \right)g}}{{A\left( {m + 2m} \right)}}\left( {M = 2m\,given} \right)$
$ = \frac{{4{m^2}}}{{3mA}}g = \frac{{4mg}}{{3A}}$