A function from a set to a set is a relation that assigns to each element in the set exactly one element in the set . The set is the domain (also called the set of inputs) of the function and the set contains the range (also called the set of outputs).
The ordered pairs has been plotted below. As you can see, the set B is the only relation that meets the requirements of the concept of function because that relation assigns each element in the set exactly one element in the set . For the other sets, at least one element in the set matches to two or more elements in the set .