Answer
... vertical
Work Step by Step
We recognize the graph of a relation as a graph of a function
when it passes the vertical line test, which assures us that
for any given x, there is no more than one y such that (x,y) is on the graph.
In other words, any vertical line intersects the graph at most at one point.
We say "at most at one point" because not all x's must be in the domain of the relation/function.
