Separation
For a given potential difference across the capacitor plates, if the separation of the plates is increased, then the electric field strength of the uniform electric field between the plates must have decreased, as \(E=\frac{V}{d}\).
Therefore, less charge must be present on the plates, to produce an electric field of lower strength. This means that less charge is stored per unit potential difference between the plates, and so the capacitance lowers. So:
\[C\propto\frac{1}{d}\]