The correct answer is NADH.
NAD (Nicotinamide adenine dinucleotide) is a coenzyme which is involved in redox reactions. It is found in two forms in cells: NAD+ which is an oxidizing form and NADH which is reduced form. NADH is a reducing agent which donates electrons and its essential role is in the production of energy (ATP) during the cellular respiration by the mitochondria through the electron transport chain.