Answer: A. It ensures that no process executes CODE SECTION Q before every process has finished CODE SECTION P.
Explanation
In short, semaphore 'a' controls mutually exclusive execution of statement count+=1 and semaphore 'b' controls entry to CODE SECTION Q when all the process have executed CODE SECTION P. As checked by given condition if(count==n) signal(b); the semaphore 'b' is initialized to 0 and only increments when this condition is TRUE. (Side fact, processes do not enter the CODE SECTION Q in mutual exclusion, the moment all have executed CODE SECTION P, process will enter CODE SECTION Q in any order.)
Detailed explanation:-
Consider this situation as the processes need to execute three stages- Section P, then the given code and finally Section Q.
It is evident that semaphores do not control Section P hence, There is no restriction in execution of P.
Now, we are given 2 semaphores 'a' and 'b' initialized to '1' and '0' respectively.
Take an example of 3 processes (hence n=3, count=0(initially) ) and lets say first of them has finished executing Section P and enters the given code. It does following changes:-
1. will execute wait(a) hence making semaphore a=0
2. increment the count from 0 to 1 (first time)
3. If(count==n) evaluates FALSE and hence signal(b) is not executed. So semaphore b remains 0
4. signal(a) hence making semaphore a=1
5. wait(b) But since semaphore b is already 0, The process will be in blocked/waiting state.
First out of the three processes is unable to enter the CODE SECTION Q !
Now say second process completes CODE SECTION P and starts executing the given code. It can be concluded that it will follow the same sequence (5 steps) as mentioned above and status of variables will be:- count = 2 (still count<n), semaphore a=1, semaphore b=0 (no change)
Finally the last process finishes execution of CODE SECTION P.
It will follow same steps 1 and 2 making semaphore a=0 and count = 3
3. if(count==n) evalueates TRUE! and hence signal(b) is executed marking semaphore b = 1 FOR THE FIRST TIME.
4 and 5 will be executed the same way.
Now the moment this last process signaled b, the previously blocked process will be able to execute wait(b) and the very next moment execute signal(b) to allow other blocked/waiting process to proceed.
This way all the processes enter CODE SECTION Q after executing CODE SECTION P.