Use the vertical line test.
Think of a vertical line moving from left to right across the graph.
If the line touches more than one point on the graph at a time, it is NOT a function.
If the vertical line touches at most one piont ion the graph, then it is a function.
Look at the circle, on the top right.
As the vertical line moves to the right, it will touch two points of the circle; therefore, the circle is not a function.
Only one graph passes the vertical line test, and it is the bottom right graph.
