To find the function,
draw a vertical line through every point. If there is no other point lies on this line, then this is a function.
If we do this vertical line test,
in the first graph, there are two points at (-1, -1) and (-1, 3) lie on the same line and therefore, it is not a function.
In the second graph, points at (-2, -2) and (-2, 1) lie on the same line and therefore, it is not a function.
In the third graph, points at (0, -1) and (0, 2) lie on the same line and therefore, it is not a function.
In the fourth graph, in each vertical line, only one point lies and hence it is a function.
