Which set of ordered pairs represents a function

A) (2,-8), (7,6), (7,0), (8,4)
B) (-8,-4), (-2,-7), (4,7), (-2,-8)
C) (-9,0), (-1,-1), (4,-2), (-1,-5)
D)-1,-4), (-4,-6), (8,-5), (-2,-4)

The set of ordered pairs that represents a function is:

C) (-9,0), (-1,-1), (4,-2), (-1,-5)

In this set, each x-value (first number in each pair) corresponds to a unique y-value (second number in each pair), which is the definition of a function.