A parallel plate capacitor has area of each plate A, the separation between the plates is d. It is charged to a potential V and then disconnected from the battery. The amount of work done in filling the capacitor completely with a dielectric constant k is :

a. 1ε₀AV²/2 d [1-1/k²]
b. 1V²ε₀A/2 kd
c. 1ε₀AV²/2 d [1-1/k]
d. 1V²ε₀A/2 kd