b
Capacitance of a parallel plate capacitor

\(C=\frac{\varepsilon_{0} A}{d}\)     \(...(i)\)

Also capacitance = potential difference\(/\)charge     \(.....(ii)\)

When battery is disconnected and the distance between the plates of the capacitor is increased then capacitance increases and charge remains constant.

since capacitance \(=\) potential difference\(/\)charge

\(\therefore\) Potential difference increases.