Please help! I’ll give brainliest! Which relation is a function?

(1, 2), (2, 3), (3, 4), (4, 5)
(1, 2), (3, 4), (5, 6), (1, 8)
(0, 2), (0, 4), (0, 6), (0, 8)
(1, 2), (2, 1), (1, 3), (3, 1)