Iron, and sulfur form the reactive portion of nitrogenase, a biological catalyst used by nitrogen-fixing organisms. fe co zn cd mo

Molybdenum is correct answer.
Nitrogenase are enzymes responsible for transformation of nitrogen to ammonia. Iron and molybdenum form MoFe protein that is responsible for producing electrons needed for reducing nitrogen (N₂) to ammonia (NH₃). Also, there is complex proteins that contains iro, silfur and molybdenum.

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