Nitrogen is an important constituent of

Options

A. Lipids
B. Carbohydrates
C. Polyphosphates
D. Proteins.

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the significance of nitrogen in biological molecules. We need to identify which of the provided options contains nitrogen as a key component.

2. **Analyzing the Options**:
- **Lipids**: These are primarily made up of carbon, hydrogen, and oxygen. Nitrogen is not a significant component of lipids.
- **Carbohydrates**: These are composed mainly of carbon, hydrogen, and oxygen as well. Nitrogen is not a key component here either.
- **Polyphosphates**: These are primarily made of phosphorus and oxygen. Nitrogen is not a significant part of polyphosphates.
- **Proteins**: Proteins are made up of amino acids, which contain nitrogen in the form of amino groups.

3. **Identifying the Role of Nitrogen**: Nitrogen is a crucial element found in amino acids, which are the building blocks of proteins. Each amino acid contains an amino group (-NH2) that includes nitrogen.

4. **Conclusion**: Since proteins are composed of amino acids and nitrogen is a vital part of the amino group in these amino acids, we conclude that nitrogen is an important constituent of proteins.

5. **Final Answer**: The correct answer is **option 4: proteins**.