Final answer:
NADP+ is reduced to NADPH in the light-dependent reactions of photosynthesis when it gains electrons from water molecules split in photosystem II, and then the electrons are transferred via the chloroplast electron transport chain to photosystem I, which catalyzes the reduction.
Step-by-step explanation:
The reduction of NADP in the light-dependent reactions of photosynthesis occurs when NADP+ gains electrons and protons to form NADPH. This process happens primarily in photosystem I (PSI), where energy from sunlight is used to excite electrons that are then transferred to NADP+ to reduce it to NADPH. The initial electrons for this process come from water molecules, which are split in photosystem II (PSII) reaction center, releasing electrons, protons, and oxygen as a byproduct.
As electrons move along the chloroplast electron transport chain, they create an electrochemical gradient conducive to ATP synthesis. Ultimately, the energy in NADPH, along with ATP, is used in the Calvin cycle to reduce carbon molecules, continuing the process of photosynthesis. Therefore, the statement that best describes the reduction of NADP+ in the light-dependent reaction is 'NADP gains electrons and protons from water molecules,' which corresponds to option A.