(c)
\(P E=-\frac{G M M}{\gamma}\)
\(T^2 \propto \gamma^3\)
\(P E=\frac{C}{\gamma}-(1)\)
\(T^2=kr^3\)
\(T^2=K\left(\frac{C}{P E}\right)^3\)
\(T^2=\frac{A}{(P E)^3}\)
\((P E)^3= \frac{A}{T^2}\)
\(P E=\frac{A^{1 / 3}}{T^{2 / 3}}\)
\(P E \propto \frac{1}{-T^{2 / 3}}\)
\(P E \propto T^{\frac{-2}{3}}\)