<span>Atoms were not indivisible and indestructible.</span>
Valency is the number of electrons present in the outermost shell.
The answer is oxidation.
That is in the redox fueling reaction,
succinate + NAD ↔fumarate + NADPH, the succinate molecule is undergoing oxidation.
As succinate molecule is providing electrons to NAD, so that it can be reduced from NAD to NADPH. So it is losing electrons and undergoing oxidation.
So the answer is oxidation.
