Answer:
a. wait , signal
Step-by-step explanation:
Semaphore is defined as a variable which is a non-negative and also shared between threads. The semaphore is used as a signaling mechanism, as well as a thread which is waiting on the semaphore and can be signaled by some another thread.
Two atomic operations are used by the semaphores. They are 1) wait, and 2) signal for the process synchronization.