Which relation is a function?

a.{(-4,3), (-2,3), (-1,2), (2,5), (3,2)}

b. {(-4,1), (-2,3), (-2,1), (-1,5), (3,2)}

c. {(-4,1), (-2,3), (-1,2), (3,5), (3,2)}

d. {(-4,1), (-2,3), (-1,1), (-1,5), (3,2)}