b
\(\Delta \mathrm{Q}_{\mathrm{L}}=\frac{\mathrm{Q}^{2}}{2 \mathrm{C}}-\left[\frac{(\mathrm{Q} / 2)^{2}}{2 \mathrm{C}} \times 2\right]=\frac{\mathrm{Q}^{2}}{4 \mathrm{C}}\)

\(=\frac{1}{4} \mathrm{CV}^{2}\)

\(=\frac{1}{4} \times 60 \times 10^{-12} \times 4 \times 10^{2}\)

\(=6 \mathrm{nJ}\)