Which relation defines y as a function of x?

A function is a relation in which each input has only one output. In the relation, y is a function of x, because for each input x (1, 2, 3, or 0), there is only one output y.

Step-by-step explanation:

For every x there is exactly one y value. We can forgive a function if some values of x do not have a y, but if there is more than one y for even one value of x, then the relation is not a function.