Explanation

(b) In capacitor, energy is stored in electric field between the plates.
Increase in energy
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \mathrm{\Delta}U = U_{f} - U_{i}$
$= \frac{1}{2}CV_{f}^{2} - \frac{1}{2}CV_{i}^{2} = \frac{1}{2}C(V_{f}^{2} - V_{i}^{2})$
Given, $C = 6\mu F = 6 \times 10^{- 6},\ V_{i} =$10 volt,
$V_{f}\ \ = 20$volt
$\therefore\ \ \ \mathrm{\Delta}U = \frac{1}{2} \times 6 \times 10^{- 6}\lbrack(20)^{2} - (10)^{2}\rbrack$
$= 3 \times 10^{- 6} \times 300 = 9 \times 10^{- 4}\ J$