Explanation/Reference:
Explanation:
There are three general factors that are used for authentication:
Something a person knows.

Something a person has.

Something a person is.

Two-factor authentication requires two of the three factors to be part of authentication process.
Incorrect Answers:
A: RSA encryption uses integers with exactly two prime factors, but the term "two-factor authentication" is not used in that context.
B: Measuring hand geometry twice only provides one factor.
C: Single sign-on (SSO) technology allows a user to enter their credentials once to gain access to multiple systems. Two-factor authentication could be used for SSO, not the other way around.
References:
Harris, Shon, All In One CISSP Exam Guide, 6th Edition, McGraw-Hill, 2013, pp. 162, 163, 207, 815