Explanation

(a) The energy which is stored in the condenser is given by
$E = \frac{1}{2}.\frac{q^{2}}{C}$ …(i)
where $q$ is charge and $C$ the capacitance.
Also, $C = \frac{\varepsilon_{0}A}{d}$ …(ii)
From Eqs. (i) and (ii), we get
$E = \frac{1}{2}.\frac{q^{2}d}{\varepsilon_{0}A}\ \Rightarrow E \propto d$
When plate separation $(d)$ is increased energy increases.