Final answer:
Symbiotic nitrogen fixation involves the conversion of nitrogen gas (N₂) to ammonia (NH₃), a process essential to the nitrogen cycle and carried out by nitrogen-fixing bacteria.
Step-by-step explanation:
Among the options provided, the occurrence of symbiotic nitrogen fixation is where nitrogen gas (N₂) is converted to ammonia (NH₃). This process is a critical part of the nitrogen cycle, and it mainly involves nitrogen-fixing bacteria that live in a symbiotic relationship with the root nodules of legumes or independently in the soil. The correct answer to the student's question is a) Conversion of nitrogen gas to ammonia, which corresponds to the process known as nitrogen fixation.
In the context of the nitrogen cycle, nitrogen fixation is the only process that directly incorporates atmospheric nitrogen gas into biologically available forms. Nitrification actually involves the oxidation of ammonia to nitrate ions, not to ammonium ions, and therefore, any statement indicating otherwise is false. In contrast, denitrification is the process where denitrifying bacteria convert nitrates or nitrites back into nitrogen gas, releasing it into the atmosphere. Lastly, ammonification is the breakdown of organic nitrogenous matter into ammonium ions (NH₄⁺).