Answer: option B. each input value is mapped to a single output value.
Justification:
A function (per definition) is an unambiguos rule that maps each input to a different output.
That means that a single input value cannot have two (or more) different images (output values of the function).
Then, the graph of a function cannot have two different points with the same x-coordinate.
For example, these two points (0,5) and (0,6) cannot belong to a same function, because that is relating the same input (0) to two different values (5 and 6) which is precluded by the definition of function.
