In an electromagnetic wave the average energy density associated with electric field will be:

$\frac{1}{2} CV^2$ $\frac{q^2}{2C}$ $\frac{1}{2} \epsilon_0^2 E$ $\frac{1}{2} \epsilon_0 E^2$

$\frac{1}{2} \epsilon_0 E^2$

$u_E = \frac{1}{2} \epsilon_0 E^2$