Step 1: Note the charges of the host and impurity ions.
In the $NaCl$ lattice the cation is $Na^+$ with a single positive charge. The impurity $SrCl_2$ supplies $Sr^{2+}$, which carries a double positive charge.
Step 2: See how the impurity enters the lattice.
Each $Sr^{2+}$ ion takes the place of one $Na^+$ ion at a cation site, because the crystal can only fit a cation where a cation already belongs.
Step 3: Spot the charge imbalance created.
Replacing a $+1$ ion with a $+2$ ion adds one extra unit of positive charge to that region of the crystal, so the lattice is no longer electrically neutral.
Step 4: Restore neutrality.
To cancel that extra positive charge, a second $Na^+$ site is left empty for every $Sr^{2+}$ that enters. So each $Sr^{2+}$ removes two $Na^+$ ions, one by substitution and one as a vacancy.
Step 5: Name the type of defect formed.
The empty site sits where a positive ion should be, so it is a cationic (positive ion) vacancy.
Step 6: Eliminate the other choices.
No anion is missing, so it is not an anionic vacancy, and there is no extra metal or non metal, so metal excess and metal deficiency defects do not apply here.
\[ \boxed{\text{Cationic vacancies}} \]