Which amino acid is optically inactive?

Explanation

To determine which amino acid is optically inactive, we need to understand the concept of chirality in amino acids.

### Step-by-Step Solution:

1. **Definition of Optical Activity**: Optical activity refers to the ability of a compound to rotate plane-polarized light. This property arises from the presence of chiral centers in the molecule.

2. **Understanding Chiral Centers**: A chiral center is typically a carbon atom that is bonded to four different groups. If a molecule has one or more chiral centers, it can exist in two forms (enantiomers) that are mirror images of each other.

...