Which relation is a function

A) {(1, 2) (2, 3) (3, 4) (1, 5)}

B) {(1, 2) (2, 5) (3, 2) (4, 5)}

C) {(1, 2) (2, 3) (3, 4) (2, 5)}

D) {(1, 2) (1, 3) (3, 2) (4, 2)}