Which ordered pair could be included in the set so that the relation remains a function of x?

To have a function, you can't have the same x-coordinate more than once.
The function given above has x-coordinates 1, 2, 3, 4.
Choices B, C, and D would make the function have a repeat of an x-coordinate.
Only choice A does not repeat an x-coordinate since the x-coordinate of choice A is 0 which is not already in the function already.