How does a denial-of-service attack work?

A.
A hacker tries to decipher a password by using a system, which subsequently crashes the network

B.
A hacker attempts to imitate a legitimate user by confusing a computer or even another person

C.
A hacker prevents a legitimate user (or group of users) from accessing a service

D.
A hacker uses every character, word, or letter he or she can think of to defeat authentication

Explanation:
In computer security, a denial-of-service attack (DoS attack) is an attempt to make a computer resource unavailable to its intended users. Typically the targets are high-profile web servers, and the attack attempts to make the hosted web pages unavailable on the Internet. It is a computer crime that violates the Internet proper use policy as indicated by the Internet Architecture Board (IAB).