The area of a circle of radius ‘a’ can be found by following integral

A

$$\int_{a}^{b} (a^{2}+x^{2}) dx$$

B

$$\int_{0}^{2 \pi} \sqrt{(a^{2}-x^{2})} dx$$

C

$$4 \times \int_{0}^{a} \sqrt{(a^{2}-x^{2})} dx$$

D

$$\int_{0}^{a} \sqrt{(a^{2}-x^{2})} dx$$