In Circular Queue
If Queue is full then first element should 0^{th} element and last element should be n-1^{th} element , lets assume an example if circular queue contains 5 element then :-
1st element should be 0th element or front =0 and last will be 4th element or rear =4
so according to option A :- full: (REAR+1) mod n == FRONT
(4+1) mod 5 = 0 = Front then this condition satisfies so option C and D eliminated
now we have to check 2^{nd} condition of option A whether it is correct or not
empty: $REAR == FRONT$
As we know in queue
if Front = Rear = -1 then queue is Empty
So option B is eliminated
Option A will be right option