Which command can be used to display running processes on a system?

The command that is used to display running processes on a system is "ps." This command stands for "process status" and provides a snapshot of the current processes running on the system when executed. It can show various details about these processes, such as their process ID (PID), the terminal associated with the process, CPU usage, memory usage, and the command that started the process.

The "top" command is also a valid option for displaying processes, but it provides a dynamic, real-time view of system processes, continuously updating the display. While useful, it is different from "ps," which shows a static snapshot at a particular moment.

The "free" command is specifically used to display memory usage information and does not display any running processes. It provides insights into the total amount of free and used physical and swap memory in the system.

The "kill" command is used to terminate processes by sending a signal to them and is not used for displaying running processes.

Thus, "ps" is the most direct command for obtaining a list of currently running processes, which makes it the correct answer in this case.