Energy density,$ U_{av} =\large\frac{1}{2} \in_oE_o^2$
$ =\large\frac{1}{2}$$ \times 8.85 \times 10^{-12} \times 50^2 = 1.1 \times 10^{-8} J/m^3$
Volume, $V = 10 \times 50 = 5 \times 10^{-4}m^3$
So, energy $= U_{av} \times V = 5.5 \times 10^{-12} J$
Ans : (b)