Which service usually runs on port 25?

A.
File Transfer Protocol (FTP)

B.
Telnet

C.
Simple Mail Transfer Protocol (SMTP)

D.
Domain Name Service (DNS)

Explanation:
SMTP uses port 25.
Incorrect Answers:
A: FTP uses port 21.
B: Telnet uses port 23.
D: DNS uses port 53.References:
Harris, Shon, All In One CISSP Exam Guide, 6th Edition, McGraw-Hill, New York, 2013, p. 1289