The charge flowing across the cell on closing the key k is equal to

CV CV/2 2CV Zero

CV/2

When the key is kept open, the charge drawn from the source is Q = CeqV = $\frac{C}{2}$V When the key is closed the capacitor 2 gets short circuited And C'eq = C ∴ Q' = CV charge flown through cell Q' - Q = $\frac{C}{2}$V ∴ (B) is correct choice.