Here the catch is :
count(*) always results to non null result of the subquery..In worst case it will result in 0 but not null
Hence in this query which is given , it is an example of nested query with correlation..And for this we take a value of R.C and then check it with every tuple of S for the condition which is given in the subquery..This condition evaluates to be false for each instance of S..Consequently the count(*) will be 0 for an instance of R of outer query..And this will happen with other tuples of R also..
But the thing is that the count(*) will give 0 which is considered non empty hence EXISTS clause will return true for each tuple of R..Hence all values of A of R will come in output..
Hence the correct answer is 3 here..