Plant nutrient critical for nitrogen fixation in legumes is

Answer: Option D

The plant nutrient critical for nitrogen fixation in legumes is Mo.
Molybdenum (Mo), an essential micronutrient, plays an important role in nitrogen (N) metabolism and protein synthesis in plants.
During symbiotic N fixation, Mo acts as a cofactor for nitrogenase enzymes to catalyze the redox reaction to convert elemental N into ammonium (NH₄⁺) ions.
Supply of molybdenum and Fe to bacteroids is therefore an important process and most likely a key regulatory component in the maintenance of nitrogen fixation in legumes.