An electromagnetic wave of total intensity I is propagating in...

The total intensity $I$ of an electromagnetic wave is related to its total average energy density $u$ by the equation:

$I = u c$

$\Rightarrow u = \dfrac{I}{c}$

In an electromagnetic wave, the total energy is shared equally between the electric and magnetic fields. Thus, the average energy density of the magnetic field ($u_B$) is exactly half of the total average energy density.

$u_B = \dfrac{u}{2} = \dfrac{I}{2c}$

Answer: $\dfrac{I}{2c}$