The most effective layer of security the organization could have implemented to mitigate the attacker's ability to gain further information is to implement logical network segmentation at the switches. Logical network segmentation is the process of dividing a network into smaller subnetworks or segments based on criteria such as function, role, or access level. This way, the organization can isolate the traffic and data of different segments, and limit the exposure and impact of an attack. If the attacker has installed a sniffer onto an inside computer, logical network segmentation can prevent the sniffer from capturing the traffic and data of other segments, thus reducing the information leakage. The other options are not as effective as logical network segmentation, as they either do not prevent the sniffer from capturing the traffic and data (A and B), or do not detect or stop the attack . References: CISSP All-in-One Exam Guide, Eighth Edition, Chapter 4, page
163; Official (ISC)2 CISSP CBK Reference, Fifth Edition, Chapter 4, page 173.