Yes, it does because it passes the vertical line test. We don't have to worry about the horizontal line test because it isn't a valid test. For example, let's imagine a parabola. That doesn't pass the horizontal line test, but it's still a function. This means that we can eliminate option B. We can do the same to C because a function doesn't necessarily have to be a straight line (like the parabola). That leaves us with options A and D. These both have to do with the vertical line test, so let's do it! If we place a vertical line anywhere on this graph, we will see that there are no vertical lines that intersect at 2 places with the graph. Therefore, it passes the vertical line test, and is a function. Hope this helps!
