The correct answer is C, Oxygen.
During aerobic cellular respiration, oxygen combines with hydrogen ions and water is released as a by-product of respiration.
Explanation;
Cellular respiration allows organisms to release energy stored in chemical bonds of glucose, and other nutrients. The energy in glucose or other nutrients such as fats is used to produce ATP, which cells use to supply their energy needs. During aerobic respiration (in presence of oxygen), oxygen is reduced and water is produced together with carbon dioxide as by-products.