When we breathe we respire molecular oxygen (O2). At sea-level, the atmosphere is dense and so every constituent gas has its greatest concentration near the surface. Our bodies need a certain number of O2 molecules for fuel and at high altitude the air is "thinner" (i.e. less dense) which means for a given volume, such as the volume of your lungs, not as many O2 molecules will exist at high altitude as at the surface. Thus, the fact that it is difficult to breathe at high altitude is DIRECTLY related most closely to the decrease in density with height, answer (c).