(d) Speed is maximum when y=a
$\therefore\ \ \ a = a\cos\left( \omega t + \frac{\pi}{4} \right)$
$\Longrightarrow \ \ \cos{\left( \omega t + \frac{\pi}{4} \right) = 1}$
$\Longrightarrow \omega t + \frac{\pi}{4} = 0 \Longrightarrow t = - \frac{\pi}{4\omega}$