Which one of the following amino acids is an essential part of human diet?

Options

A. Glycine
B. Phyenylalanine
C. Serine
D. Aspartic acid

Explanation

To determine which amino acid is an essential part of the human diet, we need to understand the distinction between essential and non-essential amino acids.

### Step-by-Step Solution:

1. **Definition of Essential Amino Acids**: Essential amino acids are those that cannot be synthesized by the human body. Therefore, they must be obtained from the diet.

2. **Total Number of Amino Acids**: There are 20 different amino acids that the body uses to build proteins. Out of these, 9 are classified as essential amino acids.

3. **List of Essential Amino Acids**: The 9 essential amino acids are:
- Histidine
- Isoleucine
- Leucine
- Lysine
- Methionine
- Phenylalanine
- Threonine
- Tryptophan
- Valine

4. **Identifying the Options**: The question provides a list of amino acids to choose from. The options given are:
- Glycine
- Phenylalanine
- Serine
- Aspartic acid

5. **Evaluating the Options**:
- Glycine: Non-essential amino acid
- Phenylalanine: Essential amino acid
- Serine: Non-essential amino acid
- Aspartic acid: Non-essential amino acid

6. **Conclusion**: Among the options provided, **Phenylalanine** is the only essential amino acid. Therefore, it is the correct answer.

### Final Answer:
**Phenylalanine**

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