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

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$ U = \large\frac{1}{2}$$ \in_0E^2_{rms}+ \large\frac{1}{(2\mu_0)}$$B^2_{rms}$

$= \large\frac{1}{2}$$ \in_o E^2_{rms} +\large\frac{ 1}{ (2\mu_o)}$$ \bigg(\large\frac{E^2_{rms}}{c^2 } \bigg)$

$= \large\frac{1}{2}$$ \in_o E^2_{rms} +\large\frac{ 1}{ (2\mu_o)} $$E^2_{rms}\in_0 \mu_0$

$ =\large\frac{ 1}{2} $$ \in_oE^2_{rms} + \large\frac{ 1}{2} $$ \in_oE^2_{rms} = \in_oE^2_{rms}$

$= (8.85 \times 10^{-12} ) \times (720)^2 = 4.58 \times 10^{-6} J/m^3$

Ans : (b)

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