b
In parallel combination : Potential difference is same across all

\(\text { Energy }=\frac{1}{2}\left(\mathrm{C}_1+\mathrm{C}_2+\mathrm{C}_3\right) \mathrm{V}^2\)

\(=\frac{1}{2}(25+30+45) \times(100)^2 \times 10^{-6}=0.5=\mathrm{E}\)

In series combination: Charge is same on all.

\(\frac{1}{\mathrm{C}_{\text {equ }}}=\frac{1}{\mathrm{C}_1}+\frac{1}{\mathrm{C}_2}+\frac{1}{\mathrm{C}_3}=\frac{1}{25}+\frac{1}{30}+\frac{1}{45}\)

\(\frac{1}{\mathrm{C}_{\text {equ }}}=\frac{(18+15+10)}{450}=\frac{43}{450} \Rightarrow \mathrm{C}_{\text {equ }}=\frac{450}{43}\)

\(\text { Energy }=\frac{Q^2}{2 C_1}+\frac{Q^2}{2 C_2}+\frac{Q^2}{2 C_3}\)

\(=\frac{Q^2}{2}\left[\frac{1}{C_1}+\frac{1}{C_2}+\frac{1}{C_3}\right]\)

\(\frac{\left(V \times C_{\text {equ }}\right.}{2} \times \frac{1}{C_{\text {equ }}}frac{V^2 C_{\text {equ }}}{2}\)

\(\frac{(100)^2}{2} \times \frac{450}{} \times 10^{-6}\)

\(\Rightarrow \frac{4.5}{86}=\frac{9}{x} E=\frac{9}{x} \times 0.5 \Rightarrow x=86\)