(d)
Given that, $\frac{dV}{dt} = k$ (say)
$\because\ \ \ V = \frac{4}{3}\pi R^{3}$
$\Rightarrow \ \ \ \frac{dV}{dt} = 4\pi R^{2}\frac{dR}{dt}$
$\Rightarrow \ \ \ \frac{dR}{dt} = \frac{k}{4\pi R^{2}}$
$\Rightarrow$ Rate of increasing radius is inversely proportional to its surface area