Which four conditions must hold for a deadlock to occur in a system with multiple processes sharing resources?

• A. Mutual Exclusion, Hold-and-Wait, Preemption, Circular Wait
• B. Mutual Exclusion, Hold-and-Wait, No Preemption, Circular Wait
• C. Mutual Exclusion, No Hold-and-Wait, No Preemption, Circular Wait
• D. Mutual Exclusion, Hold-and-Wait, No Preemption, No Circular Wait

Answer: (B)

Deadlock happens only when four conditions are present at the same time: mutual exclusion (resources are held by a single process at a time), hold-and-wait (a process holding at least one resource and waiting to acquire additional ones), no preemption (resources cannot be forcibly taken away), and circular wait (a cycle of processes where each holds a resource the next process in the cycle is waiting for). The option that lists all four matches exactly what must be true for a deadlock to occur. If any one of these conditions is removed—for example, allowing preemption so a resource can be taken away, or prohibiting a process from waiting while holding a resource, or breaking the circular chain—deadlock cannot happen. A classic picture is two processes each holding a resource and waiting for the other, forming a loop, which requires all four conditions to sustain.