Which firewall chain would be used to block a client's MSN traffic on a router?

The appropriate firewall chain to use for blocking a client's MSN traffic is the Forward chain. This chain is responsible for handling packets that pass through the router, meaning any traffic that is not destined for the router itself but is being routed to another destination.

When a client initiates an MSN connection, the traffic is generally not aimed at the router but is instead going to an external server. As such, the Forward chain comes into play, allowing the router to inspect and filter this outgoing and incoming traffic based on the rules set in the firewall. Using the Forward chain to apply rules specific to MSN traffic enables the router to manage and block that specific type of communication effectively.

The Input chain deals with packets destined for the router itself, while the Output chain is concerned with packets generated by the router. The Static option is not relevant in this context, as it refers to static NAT rules rather than firewall filtering. Therefore, the Forward chain is correctly identified as the one that will be used to control and block MSN traffic effectively.