A confidential number used as an authentication factor to verify a user’s identity is called a:

A.
PIN

B.
User ID

C.
Password

D.
Challenge

Explanation:
Personal Identification Number (PIN) is a numeric password shared between a user and a system, which can
be used to authenticate the user to the system.
Incorrect Answers:
B: User ID is used for identification, not authentication.
C: A password is a word or string of characters used for user authentication.
D: Challenge-response authentication involves one party presenting a question (“challenge”) and another party
providing a valid answer (“response”) to be authenticated. It does not specifically be a number sequence.

Harris, Shon, All In One CISSP Exam Guide, 6th Edition, McGraw-Hill, 2013, p. 162
https://en.wikipedia.org/wiki/Personal_identification_number
https://en.wikipedia.org/wiki/Password
https://en.wikipedia.org/wiki/Challenge-response_authentication#Cryptographic_techniques