Final answer:
Nitrogen gas (N2) is not a reservoir in the nitrogen cycle.
Step-by-step explanation:
The correct answer is d. Nitrogen gas. Nitrogen gas (N2) is not a reservoir in the nitrogen cycle. It is the main source of nitrogen, but it is not readily available for organisms to use. Instead, nitrogen gas must be converted into other forms, such as ammonia, nitrate, and nitrite, through processes like nitrogen fixation and nitrification.