Graph M represents a function.
Remember that a function can only take on one output for each input.
You could do the Vertical Line Test on each of the given graphs to see if they represent a function. If a vertical line intersects the graph in all places at exactly one point, then the relation is a function.
Attached is the edited screenshot of your given set of graphs (sorry if it’s a bit sloppy, I’m using my phone to post this answer).
I drew vertical lines (in blue), and the small red marks in each line indicate the areas where each vertical line crosses the graph.
As the graphs show, graph M is the only graph that has 1 point in each vertical line. This implies that each input only has 1 corresponding output.
Therefore, graph M is a function.
