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 .

A. acetyl-CoA
B. NAD O
C. pyruvate
D. NADH

D- NADH

Explanation:

Nicotinamide Adenine dinucleotide, is a co-enzyme which get reduced with hydrogen atoms from Kreb;s Cycle. Flavin Adenine   dinucleotide  is another co-enzymes(FADH2),

Generally, NADH2 transports hydrogen atoms into the matrix of mitochondria.

The hydrogen is spit to protons and electrons.

The protons are pumped by  the proton motive force(PMF) from the matrix into the intramembrane spaces.

The high concentration of  protons sets up electrochemical gradients, which supplies  the energy  needed for ATPs synthesis by ATPase enzyme, as the proton diffuses down the electrochemical gradient back into the matrix.

Therefore the energy from glucose is inform of  NADH, and this has been harnessed , for ATPs synthesis.