Which set of ordered pairs could represent a function? {(1, 121), (1, 117), (2, 113)} {(0, 23), (2, 21), (2, –4)} {(99, 98), (97, 96), (93, 96)} {(0, 12), (0, –8), (0, –128)}

The ordered are represented by (a,b) where a∈A and b∈B. A and B are some sets. 
Here, a is said to be the domain and b is said to be the co-domain. A relation is a function if for every a∈A there exists a unique image b∈B. Every element in set A must have a image in set B. If an element in set A has more than one image then that relation does not represent s function.
In the given question, the relation {(99, 98), (97, 96), (93, 96)} is a function because for every a there exists a unique image b.