b
Volume of the sphere $V=\frac{4 \pi}{3} R^{3}$
$\Longrightarrow \quad \frac{\delta V}{V}=\frac{3 \delta R}{R}$
Pressure exerted on the sphere by the piston $\quad P=\frac{F}{A}=\frac{M g}{A}$
Bulk modulus $\quad K=-\frac{P}{\delta V / V}$
OR $\quad K=-\frac{M g / A}{3 \delta R / R}$
$\Longrightarrow \quad \frac{\delta R}{R}=-\frac{M g}{3 A K}$