The correct statement regarding F-centre is:
A.Electrons are held in the voids of crystals
B.F-centre produces colour to the crystals
C.Conductivity of the crystal increases due to F-centre
D.All of the above

Hint: F – centre is a crystallographic which is short for Farbe centre. In the language this word is derived from, i.e. German, the original term used ‘Farbzentrum’ where Farbe means colour while zentrum means centre. Hence the name literally means ‘colour centre’

Complete step by step answer:
-F-centre is a kind of crystallographic defect and is observed often in various ionic solids.
In an ionic solid, anions and cations are spatially arranged according to their molecular structures and exist as a continuous cluster of anions and cations. But some defects occur as far as these spatial arrangements of anions and cations are in question. One such defect is F – centre.
-In F – centre defect, within the given crystal lattice, in the position where an anion is to be placed, we can observe one or more unpaired electrons. To put it in simpler terms, the anionic vacancy in a given crystal lattice, is occupied by electrons. This defect tends the crystal to absorb certain wavelengths of the visible spectra, i.e. it gives the crystal colour. Also, the presence of the free electrons in the voids improves the conductivity of the crystal.

Hence, Option D is the correct option.

Note:
F – centres are paramagnetic in nature and are often studied using electron paramagnetic resonance techniques. Also, a higher number of F – centres renders to a more intense colour for the crystal.