Once the process is allocated CPU and executing, which of the following events…

Answer: The only event that can occur while the process is executing is creating subprocesses and waiting for their termination. The other three described transitions are not valid.

• Issuing an I/O request and then being placed in the Ready queue is not possible: when a process issues an I/O request it becomes blocked and moves to the Waiting (blocked) state, not to Ready.

• Time-slice expiry causing the process to join the Waiting queue is not possible: when a time slice expires the process is preempted and returned to the Ready queue so it can compete for the CPU again.

• Being forcibly removed from the CPU and put in the Waiting queue due to an interrupt is not the normal transition: interrupts typically preempt the process and place it in the Ready queue unless the process explicitly performs a blocking operation that moves it to Waiting.

• Creating new subprocesses and waiting for their termination is possible: a running process may spawn subprocesses and block (enter Waiting) while it waits for them to finish.

Conclusion: The events that could not occur while the process is executing are the three incorrect transitions described above (issuing I/O and going to Ready; time-slice expiry sending the process to Waiting; interrupt causing move to Waiting). The correct selection is the choice that lists those three events as impossible.