For command-line-based, remote connections to an Ubuntu Linux computer, what protocol should you use?

Using SSH (Secure Shell) for command-line-based remote connections to an Ubuntu Linux computer is the most appropriate choice. SSH is specifically designed for secure communication and management of remote systems, providing encrypted connections for executing commands and transferring files. This makes it ideal for administrative tasks, as it allows users to securely log in to the remote machine's command line interface.

Other options like RDP (Remote Desktop Protocol) and mstsc (Microsoft Terminal Services Client) are primarily used for graphical remote desktop sessions on Windows systems rather than command line access. RDP is designed to provide desktop access to Windows environments and does not natively support Linux systems. Similarly, mstsc is a client application that connects to RDP services, and its usage is limited to Windows environments.

VPN (Virtual Private Network) is a technology that connects networks securely over the internet. While it can facilitate secure connections to a network, it does not provide a direct command-line interface for managing remote systems, which is why it is not suitable for direct command-line access to an Ubuntu computer.

Thus, SSH stands out as the best option for achieving secure and effective command-line management of remote Ubuntu systems.