Metabolism, respiration and digestion are all complex chemical processes involving many steps, happening in 3 dimensional space within cells. Metabolism is the breaking of bonds within food molecules (like sugar) to harness energy for cellular work; respiration involves moving oxygen and carbon dioxide gases between cells to facilitate cellular metabolism and digestion involves breaking down food macro structures into smaller units suitable for metabolism.
So the answer is D - each of the three process involve multiple chemical reactions.
A substance that provides humans with their main source of energy would be; food.
Polar molecules have an unequal sharing of electrons, while nonpolar molecules share their electrons equally. water does not have an equal sharing of electrons, thus making it polar