The Answer: It relies on two independent proofs of identity. Two-factor authentication refers to using two independent proofs of identity, such as something the user has (e.g. a token card) and something the user knows (a password). Two-factor authentication may be used with single sign-on.
The following answers are incorrect: It requires two measurements of hand geometry. Measuring hand geometry twice does not yield two independent proofs.
It uses the RSA public-key signature based on integers with large prime factors. RSA encryption uses integers with exactly two prime factors, but the term “two-factor authentication” is not used in that context.
It does not use single sign-on technology. This is a detractor.
The following reference(s) were/was used to create this question: Shon Harris AIO v.3 p.129 ISC2 OIG, 2007 p. 126