The process of nitrogen-fixation by bacteria does not take place in the presence of

The correct option is B

the elemental form of oxygen

The correct option is B.

Explanation of the correct option:

1. Nitrogen fixation is the essential biological process and the initial stage of the nitrogen cycle.
2. In this process, atmospheric nitrogen is converted into ammonia by nitrogen-fixing bacterial species like Rhizobium sp.
3. This bacteria remains attached to the root nodules of legumes and converts nitrogen into nitrates and atmospheric nitrogen into ammonia.
4. The nitrogen-fixing bacteria contain an enzyme called Nitrogenase which is the conversion enzyme for fixing atmospheric nitrogen.
5. Elemental oxygen leads to a deactivation of this enzyme by oxidizing and inactivating its catalytic center.

Explanation of the incorrect options:

Option A:

1. The molecular form of oxygen is bound and not reactive.
2. Thus, it does not react with the enzyme activity of nitrogen-fixing bacteria.

Option C:

Water contains oxygen in the bound, molecular form, having no impact on the enzyme activity of nitrogen-fixing bacteria.

Option D:

The elemental form of nitrogen is fixed into the organic form.

Final answer: The process of nitrogen fixation by bacteria does not take place in the presence of the elemental form of oxygen.