What type of the following defect is produced when NaCl crystal is...

When NaCl crystal is doped with $\text{MgCl}_2$, $\text{Mg}^{2+}$ ions replace $\text{Na}^+$ ions in the crystal lattice.

To maintain electrical neutrality, one $\text{Mg}^{2+}$ ion replaces two $\text{Na}^+$ ions. One $\text{Mg}^{2+}$ ion occupies the site of one $\text{Na}^+$ ion, leaving the other site vacant, which creates a cation vacancy.

Since a foreign ion is introduced into the host crystal, this type of defect is known as an impurity defect.

Answer: Impurity defect