Which set of ordered pairs represents a function?

a
{(1, 8), (8, 1), (1, 5), (5, 1)}
b
{(2, 1), (3, 1) (4, 3), (5, 3)}
c
{(5, 1), (3, 8), (3, 7), (7, 3)}
d
{(5, 1), (5, 6), (1, 3), (9, 7)}