The coefficient $C_n$ of $\sin (n \omega t)$ in the Fourier expansion of $V(t)$ is
$\begin{align*}\frac{\omega}{\pi}\int_0^{2 \pi/\omega} \sin\left(n \omega t\right) V(t) &= \frac{\omega}{\pi}\int_0^{ \pi/...$
When $n$ is even $C_n = 0$ and when $n$ is odd $C_n$ equals
$\begin{align*}C_n = \frac{4 }{n \pi }\end{align*}$
The coefficients $D_n$ of $\cos (n \omega t)$ are
$\begin{align*}\frac{\omega}{\pi}\int_0^{2 \pi/\omega} \cos\left(n \omega t\right) V(t) &= \frac{\omega}{\pi}\int_0^{ \pi/...$
Therefore, the coefficients of $\cos (n \omega t)$ are all zero.
Therefore, answer (B) is correct.

Solution to 1992 Problem 39