The radius of a sphere is reduced by 40%. By what percent will its volume decrease?

Answer: Option D

$$-40\%\to \frac{-2}{5}$$

$$\begin{array}{rl}& \text{Decrease }\%=\frac{98}{125}\times 100\\ & =\frac{98\times 4}{5}\\ & =\frac{392}{5}\\ & =78.4\%\end{array}$$