The process of nitrogen-fixation by bacteria does not take place in the presence of (b) elemental form of oxygen.
Explanation: Nitrogen fixation requires the enzyme nitrogenase, which is very sensitive to oxygen and can be deactivated by it. Therefore, most nitrogen-fixing bacteria operate in anaerobic conditions or have mechanisms to protect their nitrogenase from oxygen.
Why other options are incorrect:
(a) molecular form of hydrogen:
While some nitrogen fixation processes involve hydrogen as a reactant, the presence of molecular hydrogen is not a direct inhibitor of nitrogen fixation.
(c) water:
Water is essential for biological processes, including nitrogen fixation. Water is not known to inhibit nitrogen fixation.
(d) elemental form of nitrogen:
Nitrogen fixation is the process of converting elemental nitrogen (N2) into usable forms like ammonia (NH3). Elemental nitrogen is the substrate for nitrogen fixation, not something that inhibits it.