A company is experiencing issues with third parties tailgating authorized users during entry to secure server rooms. Which of the following would BEST alleviate this problem?

Retinal scanners verify the identity of the person presenting credentials but do not physically prevent a second unauthorized person from entering behind the authenticated user.

A mantrap consists of a small enclosed space with two interlocked doors - the second door cannot open until the first has closed and the individual has been authenticated. This physical design forces single-file entry and prevents unauthorized individuals from following an authorized user through a secured door, directly addressing the tailgating threat.

Standard door locks require authorized access to open but do not prevent a tailgater from passing through the open door before it closes behind an authorized user.

Smart card badges authenticate users at the door but do not create a physical barrier that prevents an unauthorized person from following immediately behind an authorized badge holder.