Context Switch
A context switch is the process of saving the current state of a running process and restoring the state of a different process in order to allow multiple processes to share a single CPU. It involves saving the contents of the CPU registers, program counter, and other relevant information, and then restoring them when the process is scheduled to run again.

Periodically Triggered Context Switch
A context switch can be triggered in various situations, such as when a process voluntarily yields the CPU, a process completes its execution, or a process is interrupted by an external event. In the given options, the only choice that can periodically trigger a context switch is a hardware interrupt.

Explanation
A hardware interrupt is a signal sent to the CPU from an external device or hardware component to request attention or notify the CPU about an event. When a hardware interrupt occurs, the CPU suspends the currently running process, saves its state, and jumps to a predefined interrupt handler routine to handle the interrupt. This interrupt handler routine performs the necessary operations to handle the specific event or device, and once it is completed, the CPU restores the saved state of the previously running process and continues its execution.

Example
Let's consider an example where a computer system is running multiple processes simultaneously, and one of the processes is waiting for input from a keyboard. As the user types on the keyboard, a hardware interrupt is generated and sent to the CPU. This interrupt triggers a context switch, suspending the currently running process and transferring control to the interrupt handler routine associated with the keyboard. The interrupt handler reads the input from the keyboard and performs the necessary operations. Once the input is processed, the interrupt handler routine completes, and the CPU restores the saved state of the original process, allowing it to continue its execution.

Conclusion
In summary, a hardware interrupt can periodically trigger a context switch by interrupting the currently running process and transferring control to an interrupt handler routine. This allows the CPU to handle external events or hardware devices efficiently and effectively while maintaining the execution of multiple processes in a multitasking environment.