Final answer:
The missing condition for a deadlock in the list provided is circular wait, which, when combined with mutual exclusion and no preemption, can lead to a deadlock scenario in computing systems. Therefore, the correct option is A.
Step-by-step explanation:
The student has asked about the conditions required for a deadlock to occur in a computing system. The three conditions of policy that must be present for a deadlock to be possible are mutual exclusion, no preemption, and circular wait. Hence, the correct answer is a) Circular wait. Mutual exclusion occurs when resources cannot be shared and are only available to one process at a time. No preemption refers to the scenario where a resource cannot be forcibly taken from a process that is holding it. Lastly, circular wait happens when a process is waiting for a resource held by another process, which in turn is waiting for a resource held by another process, forming a cycle of interlocked processes with each holding resources that the next process requires.