Following glycolysis and the citric acid cycle, but before the electron transport chain and oxidative phosphorylation, the carbon skeleton of glucose has been broken down to CO2 with some net gain of ATP. Most of the energy from the original glucose molecule at that point in the process, however, is in the form of

The correct answer is - NADH.

Explanation:

Glycolysis is a multistep part of cellular respiration in which food is broken down to give off energy in a form of ATP. In glycolysis, the end product is converted to Acetyl-CoA using the pyruvate dehydrogenase enzyme.

During the citric acid cycle and oxidative phosphorylation, there is NAD reduces and turns to NADH that moves to the mitochondria. It is converted to energy using redox reactions. Most of the energy from original glucose molecules are transported and exists in the forms of NAD to NADH.