The correct answer is: D. A denial of service attack.
A denial-of-service (DoS) attack is an attempt to make a machine or network resource unavailable to its intended users. A DoS attack is typically carried out by flooding the target with superfluous requests in an attempt to overload systems and prevent legitimate users from accessing them.
A virus is a malicious software program that can replicate itself and spread from one computer to another. Viruses can damage files, steal data, and even take control of your computer.
A Trojan horse is a type of malware that disguises itself as a legitimate program. Once installed on your computer, a Trojan horse can steal your personal information, damage your files, or even take control of your computer.
Cracking is the act of breaking into a computer system or network. Crackers are often motivated by a desire to steal information, damage systems, or simply to prove their technical skills.
I hope this helps!