(c) The time period of the pendulum
$T = 2\pi\sqrt{\frac{l}{g}}$
$\Rightarrow$ $T \propto \sqrt{l}$
Initially the centre of mass of the sphere is at the centre of the sphere. As the water slowly flows out of the hole at the bottom, the CM of the liquid (hollow sphere) first goes on downward and the upward. Hence, the effective length of the pendulum first increases and then decreases.