A capacitor is a system of two conductors separated by an insulating medium. Capacitance of this systems is determined purely geometrically, by the shapes, size and relative position of the conductors. The medium between the two conductor can be air or a dielectric. The way the dielectric is inserted accordingly by capacitance changes. Electric energy is stored in the capacitors. The capacitor can be connected in series or in parallel in an electric circuit.

Energy stored in between the plates of a parallel plate capacitor of plate area A, separated by distance d is.

$\frac{1}{2} \varepsilon_0 E^2 A d$

The correct answer is Option (1) → $\frac{1}{2} \varepsilon_0 E^2 A d$

Energy stored in a parallel plate capacitor per unit volume

$u=\frac{U}{V} =\frac{1}{2} \varepsilon_0 E^2$

∴  $U =\frac{1}{2} \varepsilon_0 E^2$ (volume)

$U =\frac{1}{2} \varepsilon_0 E^2 A d$