a
Let the charge of the plate is q. using the Gauss's law, the electric field of a charge sheet is \(E=\frac{q}{2 A \epsilon_0}\)

now the electric filed in between the charged plate condenser is

\(E^{\prime}=\frac{q}{2 A \epsilon_0}+\frac{q}{2 A \epsilon_0}=\frac{q}{A \epsilon_0}\)

The electric energy density is \(n_z=\frac{1}{2} \epsilon_0 E^2=\frac{1}{2} \epsilon_0\left(\frac{q}{A \epsilon_0}\right)^2=q^2 / 2 \epsilon_0 A^2\)