Answer:
A graph represents a relation that is also a function if only and if every vertical line intersects the graph at most once.
Explanation:
The vertical line test should be used to get an idea about the nature of a graph whether it represents a function or not. A vertical line must not intersect the graph multiple times as it would violate the definition of a function.
If a vertical line intersects the graph multiple times, it means the graph is having the same repeated inputs, which is against the definition of a function. A function can not have repeated input values.
Thus, we conclude that a graph represents a relation that is also a function if only and if every vertical line intersects the graph at most once.