F - centres are -(a) The electrons trapped in anionic vacancies(b) The electrons trapped in cation vaccancies(c) Non equivalent sites of stoichiometric compounds(d) All of the above.

The correct answer is (a) The electrons trapped in anionic vaccancies.
An F-center, Farbe center or color center (from the original German Farbzentrum; Farbe means color, and zentrum center) is a type of crystallographic defect in which an anionic vacancy in a crystal is filled by one or more electrons. Electrons in such a vacancy tend to absorb light in the visible spectrum such that a material that is usually transparent becomes colored. This is used to identify many compounds, especially zinc oxide (yellow).