What is the identity of the substance that has undergone net reduction after the reaction is complete?

In the context of redox reactions, a net reduction refers to the gain of electrons by a substance. In this question, the substances in consideration are NADP+, FAD, H2O2, and O2.

Oxygen (O2) is known to undergo reduction in various biological and chemical processes. For instance, in cellular respiration, O2 is reduced to form water (H2O), which signifies that it has gained electrons. In many biochemical pathways, particularly involving energy production, O2 serves as the final electron acceptor, effectively being reduced.

On the other hand, NADP+ and FAD are also molecules that can undergo reduction, but they are not necessarily the focus in all reactions where a definitive net reduction occurs. NADP+ accepts electrons to become NADPH, while FAD can become FADH2. However, whether they undergo a net reduction in a given reaction depends on the specific context.

Hydrogen peroxide (H2O2) can act as an oxidizing agent or can be further reduced, but generally, in the context of a straightforward electron transfer, it is less commonly the primary reduced species compared to molecular oxygen.

Therefore, in a reaction where a net reduction is to be identified, particularly