Answer: vertical line.
Explanation:
Ok we can define a function f(x) as "something" that maps a given input (we usually call the input "x") into a given output (we usually call the output "y")
We have one rule for functions, each input can be mapped into only one output.
So if for example, for a given value x = A.
f(A) = B
and
f(A) = C
This is mapping the input A into two different outputs, then this is not a function.
Now to the question.
If we have a vertical line (parallel to the y-axis)
This line will have only one value of x associated with it, but all the values of y.
This means that this maps one value of the input into infinite outputs, then this can not be a function.