a b c d

a

$\text{Average Energy density of Electric field is } u_e = \frac{1}{2} \epsilon_0 E_0^2$ $\text{Average Energy density of Magnetic field is } u_m = \frac{B_0^2}{2\mu_0} $ $E_0^2  = c^2B_0^2 = \frac{B_0^2}{2\mu_0\epsilon_0}$ $\Rightarrow u_e = u_m$