The correct answer is B.
OBJ-3.3: Due to the requirements provided, you should install a NIPS on the gateway router’s internal interface and a firewall on the external interface of the gateway router. The firewall on the external interface will allow the bulk of the malicious inbound traffic to be filtered before reaching the network. Then, the NIPS can be used to inspect the traffic entering the network and provide protection for the network using signature-based or behavior-based analysis. A NIPS is less powerful than a firewall and could easily “fail open” if it is overcome with traffic by being placed on the external interface. The NIPS installed on the internal interface would also allow various content types to be quickly blocked using custom signatures developed by the security team. We wouldn’t want to place the NIPS on the external interface in the correct choice for the same reasons. We also wouldn’t choose to install a NIPS on both the internal and external connections. IP filtering on both interfaces of the router will not provide the ability to monitor the traffic or to block traffic based on content type. Finally, we would not want to rely on a NIDS on the external interface alone since it can only monitor and not provide the content blocking capabilities needed.