AThe stored energy of the capacitor increases
a
When a capacitor is charged and then disconnect battery, first the capacitance will remain constant and then slowly discharge according to time constant.
The potential between the plates is $V=\frac{q d}{A \in_{0}},$ so when increasing the between plate, the voltage will increase.
The energy stored in capacitor is $W=\frac{1}{2} C V^{2} .$ Thus, the electrostatic energy stored in the
capacitor also increases.