Which DNS record type is used to resolve an IP address to a fully-qualified domain name (FQDN)?

The correct choice is the PTR record, which stands for Pointer Record. This type of DNS record is specifically used in the reverse DNS lookup process. It links an IP address back to a fully-qualified domain name (FQDN), allowing a system to verify the domain name associated with a given IP address. This capability is significant for various purposes, including network security, logging, and troubleshooting.

For instance, when a system receives a request from an IP address and needs to determine which domain name corresponds to that address, it queries the DNS for a PTR record. If a PTR record exists for the IP address, it will return the associated FQDN, providing clarity and context for the entity connecting to the network.

Other options like the MX record, which manages mail exchange servers for handling email, the A record, which resolves domain names to IPv4 addresses, and the NS record, which indicates the authoritative name servers for a domain, serve different networking purposes and do not perform the function of mapping an IP address back to a domain name.