Which of the following would BEST alleviate this problem?

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?

A.
Retinal scanners

B.
Mantraps

C.
Door locks

D.
Smart card badges

Explanation:

Reference:
http://www.datacenterjournal.com/design/what-is-a-mantrap-and-do-you-need-one/