The correct answer is Circular Wait. Deadlock occurs in a computer system when a set of processes are blocked indefinitely, each waiting for a resource held by another process in the set. The four necessary conditions for deadlock are: Mutual Exclusion, Hold and Wait, No Preemption, and Circular Wait. While all four must be present, Circular Wait is often considered the most direct and summarizing condition that describes the cyclic dependency leading to deadlock. It means there is a chain of processes, P0, P1, ..., Pn, such that P0 is waiting for a resource held by P1, P1 is waiting for a resource held by P2, and so on, with Pn waiting for a resource held by P0.
Option A, Mutual Exclusion, is incorrect as a sole cause. Mutual Exclusion means that at least one resource must be held in a non-sharable mode. While necessary for deadlock, it alone doesn't cause it.
Option B, No Preemption, is incorrect as a sole cause. No Preemption means a resource cannot be forcibly taken from a process holding it. This is another necessary condition but not sufficient on its own.
Option C, Hold and Wait, is incorrect as a sole cause. Hold and Wait means a process holding at least one resource is waiting to acquire additional resources held by other processes. It's a key condition, but deadlock requires the circular dependency implied by circular wait.