Nitrogen is an essential constituent of :

Options

A. carbohydrates
B. fats
C. mineral salts
D. proteins

Explanation

To answer the question "Nitrogen is an essential constituent of:", we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Role of Nitrogen**:
- Nitrogen is a naturally occurring element that is crucial for growth and reproduction in both plants and animals.

2. **Understand Where Nitrogen is Found**:
- Nitrogen is primarily found in amino acids, which are the building blocks of proteins.

3. **Link Nitrogen to Biomolecules**:
- Proteins are made up of amino acids, and since amino acids contain nitrogen, it follows that nitrogen is an essential component of proteins.

4. **Evaluate the Options**:
- The options given are carbohydrates, fats, minerals, salts, and proteins.
- Carbohydrates and fats are mainly composed of carbon and hydrogen, and do not contain nitrogen.
- Mineral salts are composed of elements like chlorine, calcium, and sodium, which do not include nitrogen.

5. **Conclude the Correct Answer**:
- Since nitrogen is a key component of amino acids and thus proteins, the correct answer is **proteins**.

### Final Answer:
Nitrogen is an essential constituent of **proteins**.

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