Hi Bunuel,
I think there is some gap in the question.
S1: states "Positive difference between any two elements of set S is 2." Eg: if set S={2,4,6,8} or{1,3,5,7}or any other combination with a difference 2, then the no. of elements cannot be determined since difference between any 2 elements is 2, however number of elements are varying.
S2: states "Range is 2". this is also inconsistent because if S={1,1,2,2,3,3,3}or{1,3}or {2,2,4,4} in such a case range 2 but number of elements are varying!!
combining both the inferences we say S={1,3} or {2,4} or....... as range is 2 and positive difference is also 2. Hence I think the correct choice is C.
Kindly correct me if I am wrong.