c
The friction force on the mass $M$ due to rough surface is $f=\mu M g$
In equilibrium,
For mass $m$
$T=m g$
for mass $M$
$f=\mu M g=T \Rightarrow$
$m g=\mu M g \Rightarrow m=\mu M$
If $m g>\mu M g,$ then there would be an acceleration.
Thus, we get $m>\mu M$
