A right triangle with sides 3 cm, 4 cm and 5 cm is rotated about the side of 3 cm to form a cone. The volume of the cone so formed is :

Answer: Option A

Clearly, we have r = 3 cm and h = 4 cm
∴ Volume :
$$\begin{array}{rl}& =\frac{1}{3}\pi r^{2}h\\ & =\left(\frac{1}{3}\times \pi \times 3^2\times 4\right)\pi r^3\\ & =12\pi \text{ c}{\text{m}}^3\end{array}$$