Step 1: Understand what point defects are. Crystalline solids are rarely perfect. They contain point defects where ions are misplaced or missing from their ideal lattice sites. The common ones we must match are Frenkel defect, Schottky defect, metal excess defect and metal deficiency defect. Step 2: Recall the Frenkel defect. In a Frenkel defect a smaller ion, usually the cation, slips out of its regular site and squeezes into an interstitial hole. Density stays the same because no ion actually leaves the crystal. A classic example is $AgCl$, so item A (Frenkel) pairs with the option showing $AgCl$, which is choice 3. Step 3: Recall the Schottky defect. Here a pair of oppositely charged ions, one cation and one anion, go missing together so that electrical neutrality is preserved. This lowers density and is shown by ionic solids like $NaCl$. Schottky (item B) therefore matches the $NaCl$ type entry, choice 2. Step 4: Recall the metal excess defect. When an anion is missing and the freed electron sits in that empty hole (an F-centre), we get a metal excess defect that gives colour to crystals such as $NaCl$ heated in sodium vapour. Item C pairs with choice 4. Step 5: Recall the metal deficiency defect. Some compounds like $FeO$ show a metal deficiency where a few cations are missing and the charge is balanced by other cations in a higher oxidation state. Item D therefore pairs with choice 1. Step 6: Assemble the full matching. Collecting the pairs we obtain $A-3$, $B-2$, $C-4$, $D-1$. Comparing with the given options, this is exactly option 2. So the correctly matched answer is \[ \boxed{A-3,\; B-2,\; C-4,\; D-1} \]
