Nitrogen is NOT an essential constituents of

Carbohydrates are built only from carbon, hydrogen and oxygen, with the general formula Cn(H2O)n. No nitrogen is needed, so option D is the answer.

• Chlorophyll (A): The porphyrin ring holds four nitrogen atoms around a central magnesium ion. Nitrogen is essential here, so this is wrong.
• RNA (B): Its nitrogenous bases (adenine, guanine, cytosine, uracil) carry nitrogen, so this is wrong.
• DNA (C): Adenine, guanine, cytosine and thymine are nitrogen-rich purines and pyrimidines, so this is wrong.

The core concept is the elemental composition of biomolecules. Proteins, nucleic acids and chlorophyll are the nitrogen-carrying molecules, which is why nitrogen fixation and fertilisers matter so much in agriculture. That links to Mains questions on food security and soil health.

Trap: Students group carbohydrates with proteins as 'biomolecules' and assume they share the same nitrogen content. Carbohydrates are the simple C-H-O group; nitrogen belongs to proteins, nucleic acids and chlorophyll.