How does capacitor stored energy ? derive an expression for the energy stored in a capacitor.&

Question

$\begin{array}{1 1} (a)\; U=\large\frac{Q^2}{C} \\ (b)\;U=\large\frac{Q^2}{2C}\\ (c)U=Q^2C \\ (d) U=\large\frac{QC^2}{2}\end{array} $