A parallel plate capacitor is first charged and then isolated, and a dielectric slab is introduced between the plates. The quantity that remains unchanged is:
A charge Q
B potential V
C capacity C
D energy U

The correct option is (A) charge Q
When the capacitor is kept at a voltage, it gains charge. Now when the system is isolated, the charge present on the capacitor cannot change because of the law of conservation of charge.
Charge always remains constant in isolated systems.