Which set of ordered pairs represents a function?

{(0, 1), (1, 3), (1, 5) (2, 6)}

{(0, 0), (1, 2), (2, 4), (3, 4)}

{(0, 1), (1, 2), (2, 3), (2, 4)}

{(0, 0), (0, 2), (2, 2), (2, 4)}