Which set of ordered pairs is not a function?

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