Which of the following is a Function?

Select one:
a. { (3,3), (5,5) ,( 3,6) }
b. { (4,-1), (1,0) ,( 3,5) }
c. { (2,2), (2,2) ,( 6,5) }
d. { (5,2), (5,2) ,( 6,5) }