Answer: Choice D (bottom right corner)
Reason:
This graph passes the vertical line test. It is not possible to pass a single vertical line through more than one point on this curve. Therefore, we have a function. Each input leads to exactly one output.
In contrast, something like choice B (upper right corner) fails the vertical line test. We can draw a vertical line through more than one point on this circle. An input like x = 0 leads to more than one output. This is why we don't have a function here. Same goes for the other graphs.
A vertical line by itself fails the vertical line test. It has infinitely many outputs for that specific x value input.