The correct answer to this question is ATP.

Fermentation is the metabolic process that takes place in anaerobic environments to regenerate  for glycolysis. ATP is the main product of cellular respiration and the molecular energy of the cell. The aerobic metabolism results in a very high yield of these energies because it can use oxygen as the final electron acceptor in the electron transport chain.

Anaerobic respiration occurs when there is not enough oxygen necessary for pyruvate to be shunted to the citric acid cycle. In the absence of oxygen, lactate is formed. The purpose of forming lactate is to also regenerate , as  can be used in a cyclical fashion to continue the process of cellular respiration. When oxygen supply returns, the pyruvate will be shunted away from the production of lactate and instead go to the citric acid cycle.

Chemiosmosis is the coupling of the movement of electrons down the electron transport chain with the formation of ATP by utilizing the force of the proton gradient. Think about it this way, the creation of ATP is needed in both photosynthesis and respiration and this is why it is involved in both.

The process of glycolysis indeed occurs in the cytoplasm. Answer 'glycolysis can occur only with oxygen' is incorrect, because glycolysis can actually occur with or without oxygen, due to oxygen playing no role in glycolysis.

Answer 'glycolysis is the first step in anaerobic respiration but not aerobic respiration' is incorrect, because glycolysis is simply the first step in BOTH anaerobic respiration and aerobic respiration.

Answer 'glycolysis leads to the production of 2 ATP, 1 NADH, and 2 pyruvate' is incorrect, because the end product of glycolysis is 2 NADH, not 1 NADH.

Answer 'glycolysis involves the production of G5P or glucose-5-phosphate' is incorrect, because glycolysis involves the production of G6P, not G5P.

Since there is no more oxygen, we undergo anaerobic respiration, which in humans is lactic acid fermentation. Lactic acid does not produce any energy, so it doesn't solve the ATP crisis nor does it produce ATP. It is true that lactic acid fermentation does use up pyruvate, but so does the citric acid cycle. The importance is that lactic acid fermentation turns NADH into NAD+ so that it can be recycled into glycolysis to create ATP. 

When the body isn't getting enough oxygen, it undergoes lactic acid fermentation. The purpose of lactic acid fermentation is so that your body can generate ATP through glycolysis by turning pyruvate into lactate and in the processes regenerating NAD+. Since lactate is poisonous, we eventually have to breathe. When oxygen combines with lactate it turns into lactic acid, which in turn causes muscle soreness. 

The citric acid cycle, also known as the Kreb's cycle, occurs within the mitochondria of eukaryotic cells. In prokaryotic cells, it occurs in the cytosol.

The citric acid cycle takes place in the mitochondrial matrix.

Glycolysis takes place in the cytosol, and the electron transport chain involves both the intermembrane space and the inner mitochondrial membrane.

Pyruvate from glycolysis is transported into the mitochondrial matrix for the citric acid cycle. Energy from the citric acid cycle allows protons to be pumped to the intermembrane space. The electron transport chain involves proteins along the inner mitochondrial membrane, eventually resulting in the activation of ATP synthase due to the influx of protons along their gradient.

In cellular respiration, glucose first undergoes glycolysis and is broken down into two pyruvate molecules. As the pyruvate passes through the citric acid cycle, three molecules of  are produced. The radioactive oxygen molecules would be found in the .

 is formed when electrons removed from glucose are used to reduce .  is produced by the phosphorylation of . The oxygen in  enters the mitochondrion as gaseous molecular oxygen from the atmosphere, not from glucose. Finally,  is reduced to water in cellular respiration and serves as a reactant, rather than a product, in cell metabolism.

The Krebs cycle takes place in the mitochondrial matrix. The products of glycolysis, which takes place in the cytosol, are brought to the mitochondria for the Krebs cycle and electron transport chain. The electron carriers generated during the Krebs cycle (NADH and FADH₂) are then used in the electron transport chain, which takes place on the inner membrane of the mitochondria.