Help please. (I need a answer for this)

Which of the following relations is a function? *

A. {(1, 2), (-1, 3), (-1, 8)}
B. {(3,7), (3, 6), (5, 3)}
C. {(0, 4), (5, 6), (5, 8)}
D. {(1, 2), (2, 2), (3, 4)}