Answer is Option (A)
We need to find the area of the ring first.
Area of Ring $=$ Area of Outer Circle $-$ Area of Inner Circle
$\qquad \qquad \quad =\pi b^2 - \pi a^2 = \pi (b^2-a^2)$
Now From this ring area we have to subtract the small '$n$' circles
Area of $n$ Circles $ = (\pi d^2/4) \ast n = (\pi (b-a)^2 /4)\ast n$
The Required Unpainted Area $=$ Ring Area $-$ Area of Small '$n$' such Circles
$\qquad \qquad \quad =\pi (b^2 - a^2) - (\pi d^2/4)*n$
$\qquad \qquad \quad = \pi [(b^2-a^2)- n/4(b-a)^2 ]$