Which set of ordered pairs represents a function?

Select one:
a. {(0, 1), (1, 3), (1, 5), (2, 8)}
b. {(0, 0), (1, 2), (2, 6), (2, 8)}
c. {(0, 0), (0, 2), (2, 0), (2, 4)}
d. {(0, 2), (1, 4), (2, 6), (3, 6)}