Which of the following default ports needs to be open on a firewall for a DNS server to function?

Port 23 is assigned to the Telnet protocol for remote command-line access, not DNS.

Port 53 is the IANA-assigned well-known port for the Domain Name System protocol, used over UDP for standard queries and over TCP for zone transfers and large responses. Without port 53 open on the firewall, DNS clients cannot send queries to the server and the server cannot return responses, completely breaking name resolution.

Port 68 is the DHCP client port used by hosts to receive IP address assignments from a DHCP server, not DNS.

Port 80 is assigned to HTTP for unencrypted web traffic, not DNS name resolution.