a
(a) Total electric field between the plates of the capacitor$:$
$E_{T}=\frac{V}{d}$
Then, electric field due to only one plate$:$
$E_{1}=\frac{V}{2 d}$
$\therefore$ force of one plate on another$:$
$F=E_{1} \times Q F=E_{1} \times C V=\frac{V}{2 d} \times C V=\frac{C V^{2}}{2 d}$