Choice "D" is correct. The function of a physical access control is to separate unauthorized individuals from computer resources. Examples are locks on doors to computer rooms, etc. which limit physical access to computer resources to people who need such access in the performance of their job responsibilities.
Choice "c" is incorrect. The function of a physical access control is not to protect systems from the transmission of Trojan horses. Trojan horses are software, and physical access controls would not have anything to do with them.Choice "b" is incorrect. The function of a physical access control is not to provide authentication of users attempting to log into the system; that would be done by some kind of a security system.Choice "a" is incorrect. The function of a physical access control is not to minimize the risk of incurring a power or hardware failure. A physical access control will do nothing to minimize the risk of power or hardware failures.