I believe the answer is 4 carbons. Glycolysis involves break down of glucose to two molecules of pyruvic acid (3 carbons) under aerobic conditions. At the end of glycolysis the two pyruvate molecules undergoes pyruvate oxidation to capture the remaining energy in the form of ATP. A carboxyl group is removed from pyruvate and released in the form carbon dioxide, leaving a two carbon molecule which forms Acetyl-CoA (2 molecules). Acetyl-CoA then serves as a fuel for the citric acid cycle in the next stage of cellular respiration.
