Answer :
Answer: Option B.
B, a, d and C.
Explanation:
Nitrogen cycle refer to pathway in which nitrogen flow from the atmosphere to the ecosystem.
Nitrogen is essential for plant growth and development . It is a components of protein and DNA.
Nitrogen fixing bacteria are microorganisms that are found in the root nodules of leguminous plants. The convert atmospheric nitrogen into ammonia.
Ammonia is easily converted to ammonium in the soil which the plants can absorb , it is a process called nitrogen fixation.
Nitrifying bacteria are found in the soil. They convert ammonia in the soil into nitrites and then nitrates by a process called nitrification.
Nitrogen compounds like nitrites, nitrates, ammonia and ammonium can easily be absorbed by plant root in the soil.
Animals eat plants and take-up nitrogen in their bodies. When plants and animals die, they decay by the activities of decomposer and denitrifying bacteria convert nitrates in the soil to nitrogen gas, which return back to the atmosphere.