(c)

If acceleration of the system is a then $P=(M+m) a$ from the reference frame of wedge

Component of ma along the inclined will be $m a \cos \beta$ for the block to be in equilibrium w.r.t. wedge

$m a \cos \beta=m g \sin \beta$

$a=g \tan \beta$

hence $P=(M+m) g \tan \beta$