The basic job of a is to enforce an access control policy at the border of a network.

a. firewall
b. router
c. switch
d. access point

A. Firewall

Explanation:

A firewall is a network security device that monitors incoming and outgoing network traffic and decides whether to allow or block specific traffic based on a defined set of security rules.

Firewalls have been a first line of defense in network security for over 25 years. They establish a barrier between secured and controlled internal networks that can be trusted and untrusted outside networks, such as the Internet.  

A firewall can be hardware, software, or both.

-Cisco

looking away, slouching, not focusing

explanation: