The 3rd graph (the one on the right) is not a function as a vertical line passes through twice.
The idea of a function vs not a function is mostly important for calculus although the curriculum did not used to focus on this.  One of the reasons that Math has adopted this topic is that a calculator will not help with this type of problem.
So look at Marcel’s graph: if you draw a vertical line at x = 2, it will hit the graph twice and therefore this is not a function.  Do you hear Staples saying “That was easy”?

So Marcel is not correct as this is not a function as it fails the vertical line test!
Hooray you just got 2 more points!!!