The energy stored in a capacitor of capacitance $C$ charged to voltage $V$ (holding charge $Q$) is

1. Charging transfers charge against a rising potential; the work stored is $U = \dfrac{1}{2}QV$. 2. Using $Q = CV$: $U = \dfrac{1}{2}CV^2 = \dfrac{Q^2}{2C}$. 3. Option (a) is the standard form; note $\dfrac{1}{2}QV^2$ and $\dfrac{Q}{2C}$ are dimensionally wrong for energy. _Source: Samacheer Kalvi Class 12 Physics Vol.1, Unit 1 'Electrostatics', Long Answer Q.19_