Which can be used to prevent a user program from never returning control to the operating system?

To prevent a user program from never returning control to the operating system, mechanisms like time-slicing in multitasking environments and watchdog timers can be employed. Time-slicing allows the operating system to periodically interrupt and regain control from running user programs. Additionally, watchdog timers can detect unresponsive programs and trigger a process termination or recovery. These techniques ensure that the OS maintains control and system responsiveness.