Which of the Following Is the Best Definition of a 'Denial

Which of the following is the best definition of a 'Denial of Service' attack?

A) Hacking into a bank computer and accessing credit card details.
B) Unauthorised impairment of data held on a computer disk.
C) Hacking into a computer system and deliberately impairing the data.
D) Causing a website to be inundated with a large volume of unwanted messages resulting in an overloading and crashing of the computer system.