A parallel plate air capacitor has capacity ' $C^{\prime}$, distance of separation between plates is 'd' and potential difference ' $V^{\prime}$ is applied between the plates. Force of attraction between the plates of the parallel plate air capacitor is: (1) $\frac{c^{2} \mathrm{v}^{2}}{2 \mathrm{~d}^{2}}$ (2) $\frac{c^{2} v^{2}}{2 d}$ (3) $\frac{\mathrm{cV}^{2}}{2 \mathrm{~d}}$ (4) $\frac{\mathrm{CV}^{2}}{\mathrm{~d}}$