Explanation/Reference:

To enable NetFlow on a Cisco router interface, the correct command is ip route-cache flow, which is entered in interface configuration mode. This command enables NetFlow switching on the specified interface. NetFlow is a feature that captures IP network traffic as it enters or exits an interface. By analyzing the data provided by NetFlow, a network administrator can determine things like the source and destination of traffic, class of service, and the causes of congestion1.

John should implement a Proactive security approach. This approach is part of the adaptive security strategy that is built on a 4-pronged approach - Protect, Detect, Respond, and Predict1. By being proactive, John can anticipate potential threats and vulnerabilities and take steps to mitigate them before they can be exploited.
This is in contrast to reactive or retrospective approaches, which deal with threats after they have occurred. The proactive approach is aligned with the Certified Network Defender (CND) program's emphasis on preparing network defenders to identify parts of an organization that need to be reviewed and tested for security vulnerabilities, and how to reduce, prevent, and mitigate risks in the network2345.
References:
* EC-Council's Certified Network Defender (CND) course outline and key features2.
* Information on the CND certification and its focus on proactive defense strategies3.
* Description of the adaptive security strategy, including the proactive approach, from the CND program1.
* Details on the protect, detect, respond, and predict approach to network security covered in the CND program4.
* Additional insights into the CND training and its emphasis on proactive security measures5.