Answer: D. Nitrogen is needed to build proteins and enzymes.
Nitrogen is an essential element which is not produced inside the plant body but the plant life and metabolism is largely dependent on this element. All plants obtain the nitrogen from the soil. Nitrogen is required by the plants for the production of chlorophyll, which is required for the process of photosynthesis, synthesis of amino acids which are the building blocks of proteins , and nucleic acids. The amino acids and nucleic acids are required by the plants for proper growth and metabolism. Several enzyme composition is also based upon the nitrogen which performs functions in the cellular and molecular metabolisms.
Therefore, according to the above information Nitrogen is needed to build proteins and enzymes is the correct option.