Well, if we've been paying attention in class, we already KNOW that the electrostatic force changes as the inverse square of the distance, and the top graph is conveniently labeled "Electrostatic Force".
But if we didn't already know that, we'd have to examine the graphs, and find the one where 'y' changes like 1/x² .
The top graph does that. After 1 unit of time, the force is 350. Double the time to 2 units, and the force should drop to 1/4 of 350 ... sure enough, it's a little less than 90. Double the time again, to 4 units, and it should drop to 1/4 of a little less than 90 ... by golly, it's down below 30.
The first graph is what an inverse square looks like. Now that you've worked out this graph, you'll know an inverse square relationship whenever you see it.