The vertical line test is used to determine whether a relation is a function or not. Therefore, the graph above must be showing a relation. From here, we can conduct the vertical line test to figure out if the graph is a function or not.
The vertical line test essentially states that a vertical line should be able to be drawn anywhere along the graph without hitting the graphed relation more than once. When looking at the given graph, we can clearly see that a vertical line would not hit the graphed relation more than once which means that the graph shown is indeed a function.
We have already stated that the graph must be a relation and must be a function. This gives us D as our answer.
