Hint: Nitrogen is the essential element on the earth and it is mostly present in the gaseous form. About 78% of the total is present in elemental nitrogen form and rest is oxygen and other gases.
Complete answer:The largest reservoir of nitrogen on the Earth is Air. Nitrogen is mostly present in the atmosphere in the gaseous form. This form of nitrogen is inert and cannot be used by any living form thus needs to be converted into usable forms. The composition of air is 78% nitrogen, 21% oxygen and rest 1% consists of all other gases like carbon dioxide, argon etc.The elemental nitrogen is first converted into ammonia by the process called as ammonification and then ammonia is converted into nitrites and then into nitrates by the process called nitrification. This nitrogen fixation takes place either by biological nitrogen fixation by the nitrogen fixing bacteria present in the soil or by lightning in the atmosphere.Presence of leguminous plants confers a process of nitrogen fixation. These nitrate compounds are then utilized by the plants which are then consumed by animals. The death of the plants and the animals again releases these compounds into soil, which are then converted into the elemental nitrogen by the process of denitrification. This nitrogen is again then fixed by the processes of biological fixation and lightning and the cycle continues.Except for the atmosphere, Nitrogen is also found to be stored in: watersheds in soil, groundwater, oceans, sediment and plant matter both in dead and living.
The correct answer is (B).