Which of the following functions is a relation?

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.

blah blah blah blah blah blah blah blah blah blah