Mr. Blackwell runs a small mall in a remote corner of his city. Recently, he has learned that someone has been stealing various items from his mall. The thefts have been occurring since three weeks. Mr. Blackwell has decided to hire someone who could curb such thefts by working undercover in the mall. Which type of private security personnel should Mr. Blackwell hire?

A.) a personal guard
B.) a security contractor
C.) a private investigator
D.) a security guard