You want to receive alerts if unusual activity is detected relating to the Web servers deployed in your perimeter network. What should you do

deploy a NIDS

Explanation:

The best thing to do in this scenario would be to deploy a NIDS which stands for network intrusion detection system. This system basically warns you when it detects any harmful data trying to bypass the perimeter network in which it has been deployed. This also refers to any form of cyber attack such as malware, DoS attacks, spyware, etc. Anything that may cause any damage or perform unwanted actions on the system are detected and raises an alarm.

that would be my answer.