The nitrogenase enzyme is inhibited by oxygen. Leghemoglobin reduces free oxygen concentration from the cytoplasm by binding to it and thus effectively maintains nitrogenase activity in the root nodules of leguminous plants.thus, op D.

Introduction:
Biological nitrogen fixation is the process by which certain microorganisms convert atmospheric nitrogen (N2) into a more usable form, such as ammonia (NH3), which can be utilized by plants. This process is vital for maintaining the nitrogen cycle and providing plants with a sufficient nitrogen supply. However, nitrogenase, the enzyme responsible for nitrogen fixation, is highly sensitive to oxygen and can be irreversibly inactivated by its presence. Therefore, mechanisms have evolved to prevent oxygen poisoning and protect nitrogenase activity.

Explanation:
The correct answer to the given question is option 'D' which states that inactivation of nitrogenase by oxygen poisoning is prevented by leghemoglobin. Let's understand how leghemoglobin helps in preventing oxygen poisoning during biological nitrogen fixation.

Leghemoglobin:
1. Leghemoglobin is a protein found in the nodules of leguminous plants, which includes crops like peas, beans, and clover.
2. It is produced by the plant and acts as an oxygen-binding molecule, similar to hemoglobin in animals.
3. Leghemoglobin is synthesized in the infected cells of the nodule and is stored in the cytoplasm as small spherical structures called leghemoglobin bodies.

Role of Leghemoglobin:
1. Leghemoglobin plays a crucial role in maintaining a low oxygen concentration within the nodules.
2. It has a higher affinity for oxygen than nitrogenase, which means it can bind and sequester oxygen molecules, preventing their diffusion to the nitrogenase enzyme.
3. By binding to oxygen, leghemoglobin reduces the concentration of free oxygen within the nodule, creating an anaerobic environment suitable for nitrogen fixation.
4. The oxygen that enters the nodule is rapidly absorbed by leghemoglobin, limiting its availability for nitrogenase poisoning.
5. Additionally, leghemoglobin also acts as a buffer, releasing oxygen to the plant cells when the oxygen concentration decreases.

Conclusion:
Leghemoglobin plays a vital role in preventing oxygen poisoning during biological nitrogen fixation. It binds and sequesters oxygen, reducing its concentration within the nodule and creating an anaerobic environment that protects nitrogenase activity. This mechanism ensures efficient nitrogen fixation and provides plants with a sustainable nitrogen supply.