Final answer:
During oxidoreductase-catalyzed reactions, an electron donor is oxidized, meaning it loses electrons and increases its oxidation state, exemplified by NADH turning into NAD+.
Step-by-step explanation:
During reactions catalyzed by oxidoreductases, an electron donor is oxidized. This is because in these reactions, the electron donor loses electrons, which is what oxidation entails. For instance, when reduced nicotinamide adenine dinucleotide (NADH) acts as an electron donor, it is oxidized to NAD+. In this process, it loses electrons and possibly protons (H+), leading to the oxidation state of the molecule being increased. This is a fundamental concept in oxidation-reduction reactions or redox reactions, where there is a transfer of electrons from one molecule, the electron donor, to another, which is the acceptor or reagent. The molecule that gains electrons is said to be reduced. Therefore, in the context of the question, the correct answer would be A) oxidized.