Spherical  rain drop evaporates  at a rate proportional to its surface area. The
differential equation corresponding to the rate of change of the radius of the
rain drop if the constant of proportionality is k>0,is 
 

        $\frac{dV}{dt}=-k4\pi r^2\mathrm{...}\left(i\right)$                       
$$\begin{gathered} ButV=\frac{4}{3}\pi r^3 \\ \Rightarrow \frac{dV}{dt}=+4\pi r^2\frac{dr}{dt}\mathrm{...}\left(ii\right) \end{gathered}$$

Therefore,     $\frac{dr}{dt}=-K$        
Hence, (a) is the correct answer.