The $rms$ value of the electric field of the light coming from the sun is $720\: N/C$. The average total energy density of the electromagnetic wave is

Question

$\begin {array} {1 1} (a)\;3.3 \times 10^{-3} J/m^3 & \quad (b)\;4.58 \times 10^{-6}J/m^3 \\ (c)\;6.37 \times 10^{-9} J/m^3 & \quad (d)\;81.35 \times 10^{-12} J/m^3 \end {array}$