Final answer:
Nitrogen molecules in air are converted into nitrates and nitrites through a biological process conducted by nitrogen-fixing bacteria in soil, which is part of the natural nitrogen cycle. Therefore, the correct option is A.
Step-by-step explanation:
The nitrogen molecules present in air can be converted into nitrates and nitrites by a biological process of nitrogen-fixing bacteria present in soil. During the nitrogen cycle, these bacteria convert atmospheric nitrogen into ammonia through a process called ammonification. Ammonia can then undergo nitrification where it is transformed into nitrite and then to nitrate. Plants absorb these nitrates through their roots to synthesize organic compounds essential for growth. Therefore, the correct answer to the given question is 'a. A biological process of nitrogen-fixing bacteria present in soil.'
Nitrogen fixation is primarily carried out by microorganisms that employ enzymes known as nitrogenases, containing iron and molybdenum. The most well-known nitrogen-fixing bacteria reside in the root nodules of legumes, such as peas, in a symbiotic relationship. While option 'c' discusses industrial involvement, it is important to note that this does not describe the natural conversion process in the nitrogen cycle.