How does a denial-of-service attack work?

How does a denial-of-service attack work?

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

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

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

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