c
Displacement equation of $SHM$ of frequency $'n'$
$\mathrm{x}=\mathrm{A} \sin (\omega \mathrm{t})=\mathrm{A} \sin (2 \pi \mathrm{nt})$
Now,
Potential energy
$U=\frac{1}{2} k x^{2}=\frac{1}{2} K A^{2} \,\sin ^{2}(2 \pi n t)$
$=\frac{1}{2} k A^{2}\left[\frac{1-\cos (2 \pi(2 n) t)}{2}\right]$
So frequency of potential energy $=2 \mathrm{n}$