In a system with multiple processes and resource types, which of the following is a valid method to recover from a deadlock? 

Once a deadlock is detected, the system must break the circular wait. The most common methods involve either killing processes or taking resources away from them.

Preemption (Option B): This involves temporarily taking a resource away from a "victim" process and giving it to one of the deadlocked processes. This breaks the deadlock by allowing at least one process to complete its task and release its own resources.

Process Termination: This is the most common (though harsh) method. The Operating System terminates one or more processes involved in the deadlock to release their resources.

Rollback: The system periodically saves the state of processes (check-pointing). If a deadlock occurs, the system rolls back a process to a previous safe state and restarts it.

Why the other options are incorrect:

A, C, & D (Priority Changes): Adjusting CPU priority does nothing to solve a deadlock. A deadlock is a resource-sharing problem, not a scheduling problem. If Process A is waiting for a printer held by Process B, and Process B is waiting for a scanner held by Process A, increasing their priority won't help because neither can move forward until the physical resource is released.