A number of security breaches have taken place over the last month by persons entering a restricted area by following staff members through an entrance. During investigation of these incidents, the security team determined the subjects did not have permission to enter the area. Which of the following security concepts will BEST prevent this threat?

a$.$

Infrared sensors

b$.$

Policy$-$driven access control

c$.$

Access control vestibule

d$.$

Bollards and fencing