A process in the "ready" state is

In the operating system's five-state process model, every process moves through the New, Ready, Running, Waiting (blocked), and Terminated states as it executes. A process enters the Ready state once it has been loaded into main memory and holds every resource it needs to run except the CPU itself; it then sits in the ready queue until the CPU scheduler dispatches it.

Since a process occupies exactly one of these five states at any instant, the stem's phrase "a process in the ready state" can be matched against the offered descriptions by checking which state each one names:

1. A process that is holding the CPU and having its instructions carried out is in the Running state, not the Ready state, because a Ready process is waiting for the CPU rather than using it.

2. A process that has issued an I/O request or is waiting on some event is in the Waiting (blocked) state, not the Ready state, because it cannot proceed until that event completes, regardless of CPU availability.

3. A process that has completed execution (or been aborted) and had its resources released is in the Terminated state, not the Ready state, because a Ready process is still eligible for future execution.

4. A process that has every resource it needs except the processor, sitting in the ready queue awaiting dispatch, matches the Ready state exactly.

This matches the scheduler's state-transition rule: the short-term scheduler moves a process from Ready to Running through a dispatch event, and only a process already in the Ready state can be dispatched next — confirming that "ready to run on CPU" is the description that defines the Ready state, distinct from Running, Waiting, and Terminated.