All functions are relations, but not all relations are functions. Function cannot share an x value among the coordinates. There is only one choice above where all the x values are different. In the first choice, -1 is used as an x value twice; in the second choice, 0 is used as an x value twice; in the third choice, -2 is used as an x value twice; in the fourth choice, no x value is used more than once. So the last choice above is the only function.
