How can an architect support fingerprints as a form of identification for salesforce Authentication?

Universal containers (UC) has decided to build a new, highly sensitive application on Force.com platform. The security team at UC has decided that they want users to provide a fingerprint in addition to username/Password to authenticate to this application .

How can an architect support fingerprints as a form of identification for salesforce Authentication?
A . Use salesforce Two-factor Authentication with callouts to a third-party fingerprint scanning application.
B . Use Delegated Authentication with callouts to a third-party fingerprint scanning application.
C . Use an appexchange product that does fingerprint scanning with native salesforce identity confirmation.
D . Use custom login flows with callouts to a third-party fingerprint scanning application.

Answer: D