Solution

Pre-emptive scheduling is used when the currently running process can be interrupted and CPU can be given to another process.

Statement (a) is correct because when a process moves from Running to Ready state, CPU is taken away from it.

Statement (b) is also correct because when a process moves from Waiting to Ready state, it may have higher priority and may preempt the current process.

Statement (c) is not preemption because the process completes naturally.

Statement (d) is not a valid normal transition for preemption.

So, correct statements are:

(a) and (b)