Deadlock

Step 1: Deadlock recovery strategies.
Once a deadlock has occurred, there are several ways to recover from it: - **Process termination**: Terminating one or more processes involved in the deadlock to break the circular wait condition.
- **Resource preemption**: Forcibly taking resources away from processes to break the hold and wait condition.

Step 2: Analysis of other options.
- (2) **Non preemption of resources**: This condition is part of the deadlock model, not a way to recover from deadlock.
- (3) **Banker's algorithm**: It is a prevention algorithm to avoid deadlock but does not apply for recovering from an already occurred deadlock.
- (4) **Circular wait**: This is one of the conditions that causes deadlock, not a recovery method.

Step 3: Conclusion.
The correct method to recover from a deadlock is **process termination** (1).

Step 1: Explanation of deadlock conditions.
For a deadlock to occur, **all four necessary conditions** must hold simultaneously: 1. **Mutual Exclusion**: At least one resource is held in a non-shareable mode.
2. **Hold and Wait**: A process holding at least one resource is waiting to acquire additional resources.
3. **No Preemption**: Resources cannot be forcibly removed from processes holding them.
4. **Circular Wait**: A set of processes are waiting for each other in a circular chain.

Step 2: Conclusion.
All four conditions must be present for a deadlock to occur, meaning they **must hold simultaneously**.