Which of the following defects exhibit colour to solid alkali metal...

Solid alkali metal halides such as $NaCl$, $LiCl$, and $KCl$ exhibit characteristic colors due to a type of non-stoichiometric defect called a metal excess defect.

When crystals of $NaCl$ are heated in an atmosphere of sodium vapor, sodium atoms deposit on the crystal surface. Chloride ions (Cl^{-}$) diffuse to the surface and combine with sodium atoms to form$NaCl$. During this reaction, sodium atoms lose electrons to form$Na^{+}$ions.<br><br>The released electrons diffuse into the crystal lattice and occupy the vacant anionic sites left behind by the chloride ions. These anionic vacancies trapped with unpaired electrons are known as F-centres.<br><br>The term F-centre is derived from the German word Farbenzentrum, which translates to color center. These trapped electrons absorb specific wavelengths of energy from visible light and undergo excitation, thereby imparting a characteristic color to the crystal. For example,$NaCl$appears yellow,$LiCl$appears pink, and$KCl$appears violet.<br><br>Therefore, the F-centre defect is responsible for exhibiting color in solid alkali metal halides.<br><br>Answer: F-centre$