Your picture only showed one graph and that isn’t the graph of a function.
To answer your question, you need to find a graph that does not have two points vertically on top of each other.
In the graph I see in your picture, there are two points with an x-value of 1: (1,1) and (1,-2). That says x=1 has two y-values, which makes it NOT a function.
So look for a graph that doesn’t do that anywhere and you’ll have your answer.
Let me know if you have any follow up questions.