The fundamental building block of protein is:

Options

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

Explanation

To determine the fundamental building block of protein, we need to analyze the given options and understand the structure of proteins.

### Step-by-Step Solution:

1. **Understanding Proteins**: Proteins are large biomolecules made up of smaller units called amino acids. They play crucial roles in the body, including structural support, catalyzing biochemical reactions, and regulating processes.

2. **Identifying the Options**:
- **Nitrogen**: While nitrogen is a component of amino acids (specifically in the amino group), it is not a building block of proteins itself.
- **Amino Group**: The amino group (-NH2) is part of the structure of amino acids but does not constitute a protein by itself.
- **Carboxylic Group**: Similar to the amino group, the carboxylic group (-COOH) is also part of amino acids, not a standalone building block of proteins.
- **Amino Acid**: Amino acids are the actual building blocks of proteins. They link together to form polypeptide chains, which fold into functional proteins.

3. **Conclusion**: Based on the analysis of the options, the correct answer is that the fundamental building block of protein is **amino acid**.

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

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