Which command is used to send a process to the background?

The command used to send a process to the background is "bg." When a process is running in the foreground, it can only be controlled in that context. If you want to allow the user to continue using the terminal for other commands while a process is still running, the "bg" command is utilized. This command takes a job that has been paused (typically with a "Ctrl + Z" command) and resumes it in the background, allowing the terminal to be used for different tasks without stopping the background operation.

In contrast, "fg" brings a background process back to the foreground. "kill" is a command used to terminate processes and does not influence the state of the process in terms of foreground or background operation. "su" is used to switch users in a Unix-like operating system and does not pertain to managing process states directly. Therefore, "bg" is the appropriate command for sending a process to the background.