The correct option is C Impurity defect
Silver is monovalent while cadmium is divalent. Their ionic sizes do not differ much since being neighbours.
When AgCl is dopped with CdCl2 to balance the charge difference each doubly charged Cd2+ ion has to displace two silver monovalent ions from two cationic sites. When cadmium ions substitutes silver ions, only one of the two vacant silver ion sites will be substituted by cadmium ion and the other will remain vacant.
So, the dopping of AgCl crystals with CdCl2 results in the creation of impurity defect.