Question 42 Which one is essential for the nitrogen fixation by microorganisms? A Molybdenum B Copper C Boron D Chlorine

The answer is A. Molybdenum.

Here's the step-by-step explanation:

1. Nitrogen fixation is a process by which nitrogen (N2) in the atmosphere is converted into ammonia (NH3). Microorganisms that can fix nitrogen are important in the nitrogen cycle.

2. Nitrogenase, the enzyme that is responsible for nitrogen fixation, is a complex metalloenzyme that contains molybdenum in its active site (in most known nitrogenases).

3. Molybdenum acts as a cofactor for the nitrogenase enzyme, helping it to facilitate the conversion of nitrogen to ammonia.

4. Therefore, molybdenum is essential for the nitrogen fixation by microorganisms.

The other elements listed - Copper (B), Boron (C), and Chlorine (D) - are not directly involved in the nitrogen fixation process.