Final answer:
The force pulling the boxes downward when the man lifts them is the gravitational force. This fundamental force is experienced as the weight of the boxes and is a result of the Earth's mass attracting the mass of the boxes.
Step-by-step explanation:
The force that is pulling the boxes downward when the man lifts them is the gravitational force. This is one of the four fundamental forces of nature, and in this context, it is the force that the Earth exerts on the boxes, pulling them towards its center. Gravity is always at work on objects with mass, and we experience this force as weight. The gravitation pull of Earth is what gives objects weight and is what the man has to counteract to lift the boxes.
It is important to note that despite gravity being the weakest of the four fundamental forces, it has a significant effect on everyday life because of the large mass of the Earth. For example, a magnet can defy Earth's gravitational pull on a small object like a nail due to the electromagnetic force, which is much stronger at close range. In the broader universe, gravity holds planets in orbit around stars and moons in orbit around planets.