Answer: Choice D
This shows a list of points where the x coordinate does not repeat. This means that any given x input leads to exactly one y output.
Something like choice A has x = 6 repeated; showing that x = 6 leads to both y = 11 and y = -9 at the same time. So this is why choice A is not a function. Choice B has x = 1 repeat, so it's not a function either. Choice C has x = 19 repeated, so it's not a function.
As a visual tool, you can use the vertical line test to check each set of points. If you are able to draw a single vertical line through more than one point on the graph, then it is not a function.