Question

blank can reach very large sizes and may include thousands of machines.
trojans
spyware
worms
botnets

Explanation:

Botnets are networks of compromised machines controlled remotely. They can grow to large - scale, incorporating thousands of machines. Trojans are malicious programs that disguise as legitimate software. Spyware is used for spying on user activities. Worms are self - replicating malware but not typically characterized by forming large networks of thousands of machines like botnets.

Answer:

D. Botnets