Charge Q is given to the upper plate of an isolated parallel plate capacitor of capacitance C. The potential difference between the plates

$\frac{Q / 2}{C}$

In general, for charge Q₁ and Q₂ on upper and lower plate respectively the charge distributions on outer and inner part of the plates are shown in figure.

Here Q₁ = Q, Q₂ = 0

∴ Charge on inner side of plate are $\frac{Q}{2}$ and $-\frac{Q}{2}$ respectively.

Hence $V=\frac{Q / 2}{C}$

Hence (C) is correct choice.