
As block slides down the plane, pendulum bob is subjected to two forces.
$(i)$ Weight mg perpendicularly downwards to the horizontal.
$(ii)$ Pesudo force $m g \sin \theta$ acting upwards and parallel to the inclined plane.
Resultant of these forces is perpendicular to inclined plane $A C$. So, mean position of pendulum is perpendicular to $A C$.

