Explanation

(b) A solid sphere is rotating in free space, so there is no external torque.
$ie$, $\tau = 0$
But the rate of change of angular momentum is equal to the torque.
then $\frac{dJ}{dt} = 0$
or $dJ = 0$
or $J =$constant
But $J = J\omega$, if we increase the radius of the sphere then its moment of inertia $\left( I = \frac{2}{5}\ MR^{2} \right)$ increases. From $J = I\omega$, its angular velocity decreases.
Also we know that the rotational energy
$K_{r} = \frac{J^{2}}{2I}$
Hence, $K_{r}$ decreases as $I$ is increases.