Biological nitrogen fixation is primarily carried out by microorganisms that convert atmospheric nitrogen (N2) into ammonia (NH3). Symbiotic nitrogen fixation occurs when these bacteria live in association with plants, providing the plant with usable nitrogen in exchange for carbohydrates. The most well-known example is the symbiotic relationship between Rhizobium bacteria and leguminous plants, where the bacteria reside in root nodules. A) Gram (Chickpea), B) Pea, and C) Bean are all leguminous plants. They form symbiotic relationships with Rhizobium bacteria in their root nodules, which are highly effective in fixing atmospheric nitrogen. Thus, they are helpful in nitrogen fixation. D) Paddy (Rice) is a cereal crop and not a legume. It does not form root nodules with Rhizobium. While some rice plants can have associative relationships with other nitrogen-fixing bacteria in the rhizosphere or symbiotic relationships with cyanobacteria in flooded conditions, the mechanism and scale of nitrogen fixation are significantly different and generally much less impactful compared to the Rhizobium-legume symbiosis. Therefore, paddy is the plant among the options that is not helpful in nitrogen fixation in the same prominent way as the legumes.