I believe the answer is C
Pioneer species, I think.
Answer: a. oxygen
Basically what (/the) ETC and oxidative phosphorylation is, is the products of the Krebs cycle being oxidized and oxygen receiving electrons. And when the phosphorylation part of oxidative phosphorylation occurs is when ADP gains its third phosphate group becoming ATP.
Forgive me for this poor response I was trying to be quick and it resulted in this vague and disorganized mess. To properly explain the ETC and oxidative phosphorylation it would be best to start from the beginning and explain all the stages of Aerobic respiration. Then it would be easier to Segway in to this final stage which would definitely take 2-3 descriptive paragraphs to cover.
They have increased need to carbohydrates, protein, water and iron.
Carbohydrates is the primary source for energy and active people apparently need more energy for their daily activities.
Protein helps build muscles, even though they only need a small amount more of protein than normal people, they still require protein for muscle buildings.
Water has to be needed as they may excrete more sweat by active activities in the day time. Water can also help remove waster material from body.
Iron helps the production of hemoglobin, which is in the red blood cells that help transport oxygen. As they exercise more, they need more oxygen supply for respiration, therefore there is a need for iron supply.