If ARP=reply-only is configured on an interface, what will this interface do?

When an interface is configured with ARP set to reply-only, it only responds to ARP requests for IP addresses that are already in the ARP table, particularly those marked as static. This means it will not dynamically learn new MAC addresses on that interface through ARP requests.

Thus, the correct answer indicates that the interface will accept all IP/MAC combinations that are explicitly defined as static entries in the ARP list. In other words, any request for an IP that matches an existing static entry will receive a reply, while any request for a non-existing or dynamically learned entry would not produce a response.

This configuration is often used for security purposes, to prevent the interface from learning new addresses and thereby make it less susceptible to ARP spoofing or other forms of ARP-based attacks. The other choices involve adding new entries or accepting dynamic mappings, which are not applicable in this scenario as all of those functions are restricted when ARP operates in a reply-only mode.