The correct option is C Adding SrCl2 to molten NaCl produces impurity defect
Defect is a disruption in the periodic order of a crystalline material. It occurs in crystals to lower their energy and increase thermodynamic stability.
Defects are of two types,
1. Point defect
2. Line defect
Irregularity or deviation from ideal arrangement around a point or a particle is called point defect.
Irregularity around a line or in entire rows of lattice points is called line defects.
Defects in ionic crystals, can be introduced by adding impurities
in which the ions are in different valence state than the constituent ions of the crystal. These defects are called impurity defects.
When small amounts of strontium chloride (SrCl2) are added to molten sodium chloride (NaCl), some Sr2+ ions replace Na+ ions and occupy the sites earlier occupied by the Na+ ions. Each Sr2+ replaces two Na+ ions by occupying a site and other site remaining vacant. Here, F-centres (anionic sites occupied by unpaired electrons) are not created.
Heating NaCl in sodium vapour atmosphere gives metal excess defect.
Thus, statement (a) and (c) are true.