To determine if a graph is a function the "vertical line test is used. If any point on a graph has one x value go to two y values it is not a function.
Witht he four pictures, the two on the right use a horizontal line, so it doesn't tell at all, on the leftthe upper one is vertical but it doesn't show that one x value has two corresponding y values. So it must be the lower left one.
The lower left picture has a line at x=1, and it passes through the line of the graph at two different y values, 4 and -4. So this has two y values with one x value.
