A router is advertising multiple networks to its BGP neighbor in AS 5200 with peer...

To ensure that a router advertises only networks with a prefix mask length less than or equal to 21 to its BGP neighbor, the correct configuration is Option A. This involves applying a prefix list that permits updates for networks with the specified prefix length. The prefix list acts as a filter for outgoing route advertisements in BGP, ensuring that only routes that match the criteria are advertised to the neighbor.