Out of alanine and phenyalanine, ..........is an essential amino acid.

Explanation

To determine which amino acid is essential between alanine and phenylalanine, we can follow these steps:

### Step 1: Understand the Definitions
- **Essential Amino Acids**: These are amino acids that cannot be synthesized by the human body and must be obtained through the diet.
- **Non-Essential Amino Acids**: These are amino acids that can be synthesized by the human body and do not need to be obtained from the diet.

### Step 2: Identify the Amino Acids
- **Alanine**: This is a non-essential amino acid. The body can produce alanine from other compounds.
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