A computer program that functions normally while quietly installing malicious software on a machine is known as a:

A computer program that functions normally while quietly installing malicious software on a
machine is known as a:

A.
DDoS attack.

B.
Worm.

C.
Phishing attack.

D.
Trojan.

Explanation:

Reference:
http://computer.howstuffworks.com/trojan-horse.htm