The fundamental building block of protein is :

Options

A. nitrogen
B. `NH_2` group
C. COOH group
D. amino acid

Explanation

To answer the question "The fundamental building block of protein is:", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Proteins**:
- Proteins are large biomolecules that play critical roles in the body, including structural support, transport, and catalysis of biochemical reactions.

2. **Identifying Building Blocks**:
- The fundamental building blocks of proteins are known as amino acids.

3. **Amino Acids**:
- There are 20 different amino acids that combine in various sequences to form proteins. Each amino acid has a specific structure that includes:
- A central carbon atom (C)
- An amino group (NH2)
- A carboxyl group (COOH)
- A variable side chain known as the R group, which differs among the 20 amino acids.

4. **Linking Amino Acids**:
- Amino acids link together through peptide bonds during the process of protein synthesis, forming polypeptide chains that fold into functional proteins.

5. **Conclusion**:
- Therefore, the correct answer to the question is that the fundamental building block of protein is **amino acids**.

### Final Answer:
The fundamental building block of protein is **amino acids**.

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