Answer:
The correct answer is 4. reducing molecule
Explanation:
NADH is a coenzyme that plays an important role in making ATP during cellular respiration or oxidative phosphorylation. It stays in two forms in the cell which is NADH which is it's reduced form and NAD+ which is it's oxidized form.
So as NADH is a reduced form and it acts as the electron donor and feed electron in the electron transport chain during oxidative phosphorylation. After donating electron it gets converts into it's oxidized form which is NAD+. Therefore NADH acts as reducing molecule in cellular respiration.