c
(c)
$U=\frac{1}{2} k x^2$
$x^2=\frac{2 U}{k}$
or $x \propto \frac{1}{k}$ (Since $U$ is constant)
Also $T=2 \pi \sqrt{\frac{m}{k}}$
or $T \propto \frac{1}{\sqrt{k}}$
Therefore $x \propto T$
Hence the oscillation with maximum $x$ will have the maximum time period.