d
Charge $=\mathrm{KCV}-\mathrm{CV}=(\mathrm{K}-1) \mathrm{CV}$
Energy absorbed $=(\mathrm{K}-1) \mathrm{CV}^{2}$
Energy $=\frac{1}{2} \times \mathrm{K} \times \mathrm{C} \times \mathrm{V}^{2}=\frac{(\mathrm{K}-1)}{2} \cdot \mathrm{CV}^{2}$
Now $\frac{1}{2} \times \mathrm{K} \times \mathrm{C} \times \mathrm{V}^{2}+$ work $=\frac{1}{2} \times \mathrm{C} \times \mathrm{V}^{2}=(\mathrm{K}-1) \mathrm{CV}^{2}$
$\Rightarrow$ Work $=\frac{1}{2}(\mathrm{K}-1) \mathrm{CV}^{2}$