Which set of ordered pairs represents y as a function of x?

a
{(0, 1), (2, 4), (5, -1), (2, 5)}
b
{(1, 1), (2, 2), (3, 3), (1, 0)}
c
{(4, 1), (4, 2), (4, 3), (4, 4)}
d
{(1, 4),(2, 4), (3, 4), (4, 4)}