Final answer:
When an event that blocks a thread resolves, the thread moves to the ready queue, showing it's prepared to run when the scheduler allocates CPU time.
Step-by-step explanation:
When the event for which a thread is blocked occurs, the correct answer is a) Thread moves to the ready queue. In multitasking operating systems, threads can exist in various states: new, runnable (or ready), blocked, running, and terminated.
When a thread is waiting on a blocked event, such as I/O completion, resource availability, or acquisition of a lock, it is in the blocked state. Once the blocking event is resolved, the thread transitions from the blocked state to the ready state, indicating that it is prepared to run and is waiting for the scheduler to allocate CPU time to it. This behavior is part of a thread's lifecycle in thread management within an operating system.