When we have two sets (Domain and range) connected, we have a relation. If every element of the domain is related to only one element of the Range, the relation is a function.
In this case, A and C are functions, because every element of the domain is connected with one element of the range.
On the other hand, B is just a relation because, for example, 3 is connected with 7 and 4.
Answer: B.