Option B is the correct answer.
In case of a function each x-value is mapped to exactly one y-value. If an x value is mapped to more than one y-values the relation will not be a function. To test if a relation is a function we use vertical line test. If no vertical line crosses the graph at more than one points, then the graph represents a function. This shows no two y-values have the same x value or each input value is mapped to a single output value.
