Answer: (b) H* gradient
Explanation: The aforementioned gradient is effectively utilized by ATP synthase in the process of producing adenosine triphosphate via chemiosmosis. The presence of H2O is indispensable in establishing the H+ gradient. However, it is not invariably a requisite for photophosphorylation, given that certain photosynthetic organisms possess the ability to employ alternative electron donors. The indispensability of oxygen and NADP+ in the photophosphorylation process is not warranted.