Is this an example of newton’s first, second, or third law?

Let's look at Newton's three laws of motion.
1) An object at rest will remain at rest, and an object in motion will remain in motion, unless acted upon by an outside force.
2) Force is directly proportional to mass and acceleration (F=ma).
3) Every force on an object has an opposite reactive force in the opposite direction.

I believe the second law pertains the most to your diagram, because the three elements being considered are F, m, and a. It won't be the first because it is accelerating, and although the third is true in this system, it is not the main law in action.

This is newton second law