In a parallel plate capacitor, the region between the plates is filled by a dielectric slab. The capacitor is charged from a cell and then disconnected from it. The slab is now taken out :

Explanation

When a charged capacitor is isolated from the battery then its charge remains constant. On taking out the dielectric slab there is decrease in capacitance. Using, `U=(Q^(2))/(2C)` we can understand that energy stored in capacitor will increase for a given amount of charge when capacitance decreases.