Which protocol lets you back up router configurations using the Cisco IOS MIB?

SNMP (Simple Network Management Protocol) is the correct choice. It's used for managing and monitoring network devices. The Cisco IOS MIB (Management Information Base) is a database of network device parameters, and SNMP agents on the router allow network management systems to query and modify these parameters, including backing up configurations. ARP (Address Resolution Protocol) maps IP addresses to MAC addresses. SMTP (Simple Mail Transfer Protocol) is for sending email. CDP (Cisco Discovery Protocol) is a Cisco-proprietary protocol used to discover information about directly connected Cisco devices. None of these are designed for configuration backup via the MIB.