When several computers connected to the internet have been set up to forward spam and sometimes viruses to other computers on the internet, what is responsible?

spyware
worms
viruses
botnets

Option D: Botnet

is the correct answer.

Explanation:

A Botnet can be taken as the combination of two words "robot" and "networks". Botnets are used in depicting the malicious content as the breach the network security. They are used for stealing data by Distributed Denial of Service attack (DDoS) and by sending spam to all other devices connected to a network. All this scheme is managed by the attacker itself.

All other listed options spread malicious data by some other means.

I hope it will help you!