The nitrogen cycle .
A. indicates the process of uptake of nitrates (NO₃ minus) by plants.
B. marks transformation of ammonium (NH₄ plus) into nitrites (NO₂ minus) by bacteria.
C. marks synthesis of ammonium (NH₄ plus) by decomposers.
D. marks transformation of nitrogen (N₂) from the atmosphere to ammonium (NH₄ plus) by bacteria.
E. marks organisms that take part in transformation of nitrogen from the atmosphere (N₂) into ammonium (NH₄ plus) and are located in the root nodules.

D. marks transformation of nitrogen (N₂) from the atmosphere to ammonium (NH₄ plus) by bacteria.

Explanation:

The nitrogen cycle is a biogeochemical cycle which involves transformation of atmoshperic nitrogen into many chemical forms including nitrite (NO2), nitrates (NO3) and ammonium (NH4).

The process of nitrogen cycle starts with the process of nitrogen fixation. In this process several nitrogen fixation bacteria such as cyno bacteria converts the atmospheric nitrogen into ammonium (NH4) in the soil. The resultant ammonium (NH4) can be directly used by plants or transform into nitrites or nitrates through nitrification process.

Hence, the correct option is D.

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