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

$\frac{1}{2} L^2$ $\frac{B^2}{2 \mu_0}$ $\frac{1}{2} \mu_0 B^2$ $\frac{1}{2} \frac{q}{B^2}$

$\frac{B^2}{2 \mu_0}$

Learn this. $u = \frac{B^2}{2 \mu_0}$