Definition 1: A relation is any subset of a Cartesian product. For instance, a subset of , called a "binary relation from A to B," is a collection of ordered pairs (a,b) with first components from A and second components from B, and, in particular, a subset of
is called a "relation on A."
Definition 2: A function is a relation that associates each element x of a set X to a single element y of another set Y (possibly the same set). A function is uniquely represented by its graph which is the set of all ordered pairs (x, f (x)).
From these definitions you can see that every function is a relation from X to Y, but not via versa (because you can consider relation - here for one x exist two y).
Answer: Correct choice is B.