Answer: Choice D
Step-by-step explanation:
Assuming there are four answer choices, we can eliminate choices A through C because they are functions. This is because they pass the vertical line test.
The vertical line test is where we try to draw a single vertical line through more than one point on the curve. If such a task is possible, then it is said to "fail the vertical line test" and it's not a function.
For choice A, we cannot draw a single vertical line through more than one point on the parabola. Choice A passes the vertical line test. Hence, it is a function. The same goes for choices B and C.
Unfortunately choice D is not shown, but if it's the only thing left, then I'm assuming that it's some curve that fails the vertical line test.