b
As the capacitors are connected in parallel, therefore potential difference across both the condensors remains the same. 

\(\therefore Q_{1}=C V ;\) 

\(Q_{2} =\frac{C}{2} V \) 

\(\text { Also }, Q =Q_{1}+Q_{2}\)

\( = CV+\frac{C}{2} V=\frac{3}{2} C V\)

Work done in charging fully both the condensors is given by

\(W=\frac{1}{2} Q V=\frac{1}{2} \times\left(\frac{3}{2} \,C V\right) V=\frac{3}{4}\, C V^{2}\)