SSH (Secure Shell) is the best and most secure method for accessing a network appliance from an external location, according to the CompTIA Network+ N10-009 objectives related to secure management protocols and network security. SSH provides encrypted remote command-line access, ensuring that authentication credentials, commands, and session data are protected from interception, eavesdropping, and man-in-the-middle attacks.

Telnet is an insecure legacy protocol that transmits all data, including usernames and passwords, in cleartext, making it unsuitable for any external or untrusted network access. RDP is primarily used for graphical remote desktop access to Windows systems and is not commonly used for managing network appliances such as routers, switches, firewalls, or load balancers. Additionally, RDP introduces a larger attack surface if exposed externally. FTPS is a secure file transfer protocol and is designed for transferring files, not for interactive device management or configuration.

The Network+ N10-009 exam emphasizes the use of secure administrative access methods, and SSH is the industry-standard protocol for managing network infrastructure devices. It supports strong encryption, key-based authentication, and secure tunneling, making it ideal for remote administration over untrusted networks such as the internet.