Question

which nitrogenous compound appears last in the nitrogen cycle?
nitrate
nitrous oxide
ammonia

Explanation:

To determine the last nitrogenous compound in the nitrogen cycle, we analyze the processes: Nitrification converts ammonia to nitrate, and denitrification (the final step) converts nitrates into gaseous forms like nitrous oxide ($\ce{N2O}$) which is released into the atmosphere. Ammonia is an early form (from ammonification), nitrate is intermediate (from nitrification), and nitrous oxide is produced in the final denitrification step.

Answer:

Nitrous Oxide