On a function groupset, you can't have the same X values, but you can have the same Y values. Example: If you had {(3,1) (5,1) (6,3)} it would be a function since no X values are the same.
On choice A., they all have the same Y value, but no X values are the same. On choice B, (2,3) and (2,4) have the same X value. On choice C, (1,3) and (1,4) have the same X value. On choice D, they all have the same X value.
In short, your answer would be choice A, because no X values are the same.