(c) Velocity = acceleration
$\omega\sqrt{a^{2} - y^{2}} = \omega^{2}y$
$\sqrt{{(2)}^{2} - {(1)}^{2}} = \omega(1)$
$\Rightarrow \ \ \ \ \ \ \ \ \ \omega = \sqrt{3}$
$T = \frac{2\pi}{\omega}$
$\Rightarrow \ \ \ \ \ \ \ \ \ \ \ T = \frac{2\pi}{\sqrt{3}}$