c
Force of attraction between the plates of the parallel plate air capacitor is

\(F=\frac{Q^{2}}{2 \varepsilon_{0} A}\)

where \(Q\) is the charge on the capacitor, \(\varepsilon_{0}\) is the permittivity of freespace and \(A\) is the area of each plate.

But \(Q=C V\)

and \(C=\frac{\varepsilon_{0} A}{d}\) or \(\varepsilon_{0} A=C d \quad \therefore \quad F=\frac{C^{2} V^{2}}{2 C d}=\frac{C V^{2}}{2 d}\)