A function is one to one if no two elements of the domain (set of values of x) correspond to the same element in the range (set of values of y). In other words, each value of x has exactly one value of y and no value of y is the image of more than one value of x.
For the parabola in the graph, every value of x corresponds to only one value of y.
But the values of y repeat themselves, for example, for x=0 and x=20 the image on the range is the same y=0.
So this function cannot be considered a one-to-one function.