A autonomous agent that copies itself into one or more host programs, then propagates when the host is run, is best described as a:

A.
Trojan horse

B.
Back door

C.
Logic bomb

D.
Virus

Explanation:
A virus is a piece of software designed to infect a computer system. I can go into this further, but the answer is obvious.
Reference: Security + (SYBEX) page 76