A function is a relation in which each element of the domain is mapped to one element of the range. Graphically, this presents as a graph that will pass the vertical line test; that is, a vertical line drawn anywhere on the graph will not hit more than one point.
In order to make this true, we need to eliminate one of the two points that lie on the same vertical line. These points are (1, 1) and (1, 3). Out of these two, the only one listed as a choice is (1, 3).
