So what makes a function a function is that every input (x) has only one output (y). To test if a line is a function on a graph, you would use something called the "vertical line test". Move a vertical line across the graph, and if at any point there was more than one point on the vertical line it would not be considered a function. And the only graph that has no more than one point when moving a vertical line across the graph is the last graph.
