Recall Kepler's Third Law:
$\begin{align*}\frac{s^3}{T^2} = \mbox{ const for orbits around a given mass}\end{align*}$
where $s$ is the semimajor axis and $T$ is the period of the orbit. This equation implies that
$\begin{align*}T = C s^{3/2}\end{align*}$
where $C$ is a constant. For a circular orbit $s = R$ . Therefore, answer (D) is correct.

Solution to 2001 Problem 3