The power variation is not good for a server or any kind of computer system. Which of them is the most deadly?

A.
A surge

B.
Sags

C.
Brownouts

D.
A spike

Explanation:
A spike is a very high voltage level-usually twice the normal voltage-that is supplied for an instant (usually in the millisecond range), and can be deadly for servers.
References:
Charles J. Brooks, Server+ Certification Exam Cram 2 (Exam SK0-002), QUE Publishing, Indianapolis, 2006, pp. 227-240.