Which of the following relation is function?

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