An anabolic reaction is one in which larger molecules are made from combining smaller molecules. Even without knowing the exact mechanics of the reactions given in the answer choices, we know that we are looking for a reaction in which multiple molecules combine to form a single molecule.

Out of the options, there is only one time where a larger molecule is made by the combination of two smaller ones: when acetyl CoA (2 carbons) and oxaloacetate (4 carbons) come together in order to create citrate (6 carbons).

The generation of pyruvate from glucose results in two smaller molecules from one larger molecule; this is a catalysis reaction. The conversion of glucose-6-phosphate to fructose-6-phosphate is an isomerization reaction. The transition from citrate to ketoglutarate is processed through an intermediate, but is ultimately a catalysis reaction.

Fermentation is a catabolic process which does not require oxygen. In contrast, Krebs cycle (citric acid cycle) and oxidative phosphorylation (chemiosmosis and electron transport) do use oxygen. Aerobic respiration is much more efficient than anaerobic respiration in producing ATP.  

The Krebs cycle takes place within the mitochondrial matrix of mitochondria. Glycolysis occurs in the cell's cytosol. The stroma is part of plant chloroplasts, thus it is not the site of the Krebs cycle.

The citric acid (Krebs) cycle takes place in the mitochondrial matrix. The Krebs cycle involves using acetyl-CoA as a substrate to produce high energy electron carriers  and  to later participate in electron transport, ultimately yielding . Glycolysis is the process by which glucose is split into two molecules of pyruvate, and occurs in the cytoplasm. The electron transport chain performs its function in the inner mitochondrial membrane. The nucleus and ribosomes are not part of the citric acid cycle.

Two  molecules are produced per one "turn" around the cycle. Note that each glucose results in two pyruvate, which convert to two acetyl-CoA and power the citric acid cycle for two turns.

Glycolysis, is a series of reactions in the first part of cellular respiration. It involves the breaking down of larger molecules such as glucose into smaller ones such as pyruvate. Glucose is a six-carbon molecule that gets broken down into two three-carbon pyruvate molecules during glycolysis. Afterwards, pyruvate is converted to acetyl-CoA, which enters the Krebs cycle in the mitochondria.

Immediately after pyruvate is formed through glycolysis, it enters the mitochondria. Here, it undergoes a reaction to form the following products:

The reaction, known as the "intermediate step" or "pyruvate dehydrogenase complex (PDC)" can be summarized as:

Pyruvate is a three-carbon molecule that forms a two-carbon acetyl-CoA and a molecule of carbon dioxide.

The Krebs cycle begins with the combination of oxaloacetate with acetyl-CoA and produces .  is not product of the Krebs cycle, but a reactant.

Under anaerobic conditions, the 2 molecules of pyruvate produced from glycolysis cannot undergo further oxidation; therefore, they are reduced to lactate (lactic acid) and . This allows glycolysis to continue in order for cells to produce ATP under anaerobic conditions. Once  is cycled back through glycolysis, glucose metabolism can continue to produce 2 net molecules of ATP for cellular energy.

Upon completion of the citric acid cycle, 1 molecule of , 2 molecules of , 3 molecules of , and 1 molecule of  are produced.  is not produced during the citric acid cycle.  is the product of the pentose phosphate pathway.  is a powerful reducing agent used in several metabolic pathways. For example, it is used in red blood cells to reduce glutathione. Note that the products listed above represent those from one turn of the citric acid cycle; each molecule of glucose produces two molecules of acetyl-CoA, thus the cycle turns twice per glucose molecule.