A parallel-plate capacitor is charged and then is disconnected from the battery. By what factor does the stored energy change when the plate separation is then doubled? It becomes four times larger. It becomes one-half as large. It stays the same. It becomes two times larger.

The correct option is: It becomes one-half as large.

Explanation:

The energy stored in a capacitor is inversely proportional to the distance between plates.

If the plate separation (distance between plates) is doubled, it means the Energy stored will be halved. That is, the energy stored becames one-half as large.