A function has only one output for every input. In graph form there can only be one x value. The best way to tell when looking at a graph is to draw a vertical line. If that line hits two or more points on the graph it is not a function. Say you select the x value of 2 to draw your line. You’ll see in 3 of the 4 graphs your vertical line goes through 2 or more points whereas for the lower right graph your vertical line does not have more than one point on the graph.
