For choices A, B and D, the x values repeat in some way.
Choice A has x = 2 repeated
Choice B has x = 10 repeated
Choice D has x = 13 repeated
A function cannot have repeated x values. Put another way, a function is only possible if any given x input goes to exactly one y output.
Choice C shows each x value listed once only. If you were to graph these points, then the relation will pass the vertical line test.
