Which relation is a function?

a coordinate grid containing a u shaped graph with arrows on its ends. the u shape opens upwards and the bottom of the u shape is located at the origin.

coordinate grid with graph of a vertical line at x equals 3.

a coordinate grid containing a u shape with arrows on both ends that opens to the right. the bottom portion of the u passes through the origin.

a coordinate grid with the graph of a circle centered at the origin and passing through the point begin ordered pair 2 comma 1 end ordered pair.