The acceleration $d^{2} x / d t^{2}$ of a particle varies with displacement $x$ as $\frac{d^{2} x}{d t^{2}}=-k x$
where $k$ is a constant of the motion. The time period $T$ of the motion is equal to
(a) $2 \pi k$
(b) $2 \pi \sqrt{k}$
(c) $2 \pi / \sqrt{k}$
(d) $2 \pi / k$