(d) Let $r$ be the amplitude of oscillation and $T$ be the time period in SHM. Then total distance travelled in time $T = 4r$
$\therefore$ Average velocity, $v_{av} = \frac{4t}{T} = \frac{4r}{2\pi/\omega}$
$= \frac{2r\omega}{\pi} = \frac{2v_{\max}}{\pi}$