Auses multiple systems to attack one or more victim systems or websites with the intent of denying service to legitimate users wishing to log on or utilize the attacked server.
1. worms
2. viruses
3. trojan
4. denial-of-service attacks

4. A Denial of Service attack (DDOS attack).

Explanation:

A DDOS attack is a malicious attempt to disrupt the normal traffic to a service.

In essence, it sends an enormous amount of requests to the service, until the server is overwhelmed because it can't handle that much traffic, and collapses in an overflow.

Thus, regular users are not able to access their services.

Usually, attackers use a botnet, a network of "zombie" computers that have been previously infected with a malware that allows the attacker to remotely control them, then the botnet starts to send a flood of traffic from different locations, and make the attacker difficult to detect or track.