Which protocol allows using the copy command to back up up to 20 router configurations globally?

FTP (File Transfer Protocol) is the correct answer because it is a standard network protocol used for transferring computer files from a server to a client or vice versa over a computer network. This makes it suitable for backing up multiple router configurations to a central server using the copy command in Cisco IOS, especially when dealing with a larger number of devices like 20 routers. TCP (Transmission Control Protocol) is a foundational protocol that provides reliable, ordered, and error-checked delivery of a stream of octets between applications running on hosts communicating over an IP network. It's a transport layer protocol, not an application layer protocol for file transfer. SMTP (Simple Mail Transfer Protocol) is used for sending and receiving email. SNMP (Simple Network Management Protocol) is used for managing and monitoring network devices, not for transferring configuration files in bulk using the copy command.