Deadlock:
Deadlock is a situation where two or more processes are unable to proceed because each is waiting for the other to release a resource. In other words, deadlock occurs when there is a circular dependency between two or more processes, resulting in a state where none of the processes can complete.

Necessary conditions for deadlock:
There are four necessary conditions for deadlock to occur. These conditions are as follows:
1. Mutual Exclusion: At least one resource should be non-sharable, which means only one process can use it at a time. If a resource is already being used by one process, it cannot be used by another process until it is released. This condition ensures that only one process can access the resource exclusively.

2. Hold and Wait: A process must be holding at least one resource and waiting for additional resources that are being held by other processes. In other words, a process must have acquired some resources and is waiting to acquire more resources while holding the existing ones. This condition creates a situation where processes are dependent on each other for resources.

3. No Preemption: Resources cannot be forcibly taken away from a process unless the process voluntarily releases them. This means that once a process is allocated a resource, it cannot be forcibly removed from the process until the process releases it. This condition ensures that a process does not lose its resources while it is still executing.

4. Circular Wait: There must be a circular chain of two or more processes, where each process is waiting for a resource held by the next process in the chain. This condition creates a circular dependency among the processes, resulting in a deadlock situation.

Explanation:
Option (a) states that the necessary conditions for deadlock are non-preemption and circular wait. Non-preemption refers to the condition where resources cannot be forcibly taken away from a process. Circular wait refers to the condition where there is a circular chain of processes waiting for resources held by each other. Both of these conditions are necessary for deadlock to occur.

Option (b) states that the necessary conditions for deadlock are mutual exclusion and partial allocation. Mutual exclusion refers to the condition where at least one resource is non-sharable and can be accessed by only one process at a time. Partial allocation refers to the condition where a process is holding some resources and waiting for additional resources. While these conditions are necessary for resource allocation, they do not directly contribute to deadlock.

Therefore, the correct answer is option (c) - both (a) and (b). Both non-preemption and circular wait, as well as mutual exclusion and partial allocation, are necessary conditions for deadlock to occur.