$A$ particle of mass $m$ is executing $S.H.M.$ about the origin on the $x$-axis with frequency $f = \frac{\sqrt{Ka}}{\pi m}$, where $K$ is a constant and $a$ is the amplitude of $S.H.M.$ If $x$ is the displacement of the particle at time $t$, the potential energy of the particle will be:

• A
  $\frac{1}{2} K a x^2$
• B
  $\pi K a x^2$
• C
  $2 \pi K a x^2$
• D
  $2 K a x^2$