What is a denial of service attack?

a$.$ It is an attacked when the perpetrator sends hundreds of messages from randomly generated false addresses, overloading an Internet service provider's e$-$mail server.

b$.$ It is an attack when the perpetrathis inputting so much data that the input buffer overflows. The overflow contains code that takes control of the company's computer.

c$.$ It is an attack when the perpetrator uses software to guess company's addresses, send employees blank e$-$mails, and add unreturned messages to spammer e$-$mail list.

d$.$ It is an attack when the perpetrator is inserting malicious query in input such that it is passed to and executed by an application program.