Explain the term "F-centres" with a suitable example.

Options

1. AIt is a vacancy defect where ions are missing from their lattice positions, e.g., NaCl crystals.
2. BIt is a vacancy defect where equal number of cations and anions are missing from their lattice positions, e.g.
3. CIt is an interstitial defect where ions occupy the interstitial sites, e.g., ZnS crystals.
4. DIt is an interstitial defect where equal number of cations and anions occupy the interstitial sites, e.g., ZnS