C

In a plane electromagnetic wave, the energy densities of the electric and magnetic fields are related through the equation:
$$U_E = \frac{1}{2} U_B$$
This equation shows that the energy density of the electric field ($U_E$) is half that of the magnetic field ($U_B$).
Therefore, by substituting different relations between $U_E$ and $U_B$, we conclude that the correct proportional relation is given in Option C:
$$U_E = 2U_B$$ is incorrect,
and thus we confirm the relationship between energy densities stated in Option C is correct.