Final answer:
NADP+ is considered reduced to NADPH when it gains electrons during the photosynthetic process, following the chemical principle that reduction involves the gain of electrons.
Step-by-step explanation:
When electrons end up on NADP+ at the end of the electron transport chain (ETC), it is referred to as "Reduced". This is because the molecule gains electrons, which is the very definition of a reduction in chemistry. Specifically, NADP+ (nicotinamide adenine dinucleotide phosphate) accepts a pair of electrons and one proton (H+) during the light-dependent reactions of photosynthesis, thus being transformed into NADPH.
NADPH then carries these electrons for use in the Calvin cycle, where they help to reduce carbon compounds. The process of reduction implies gaining electrons, and since electrons are negatively charged, the recipient molecule is considered to have been "reduced" in charge. This is contrasted with oxidation, where a molecule loses electrons and thus is "oxidized".