The only optically inactive amino acid is

Glycine

The correct answer is Option (1) → Glycine.

Optical activity in molecules arises due to the presence of a chiral center, which typically means the molecule has a carbon atom bonded to four different groups.

Amino acids are usually chiral, meaning they can exist in two mirror-image forms (enantiomers) because their alpha-carbon (the carbon adjacent to the carboxyl group) is attached to four different groups: a hydrogen atom, an amino group (–NH₂), a carboxyl group (–COOH), and a unique side chain (R group).

Glycine is the only amino acid that does not have a chiral center. This is because the R group in glycine is a hydrogen atom (–H), meaning the alpha-carbon is bonded to two hydrogen atoms, making glycine achiral (not optically active).

Other amino acids, including Proline, Serine, and Lysine, have different R groups attached to the alpha-carbon, making them chiral and optically active.

Therefore, Glycine is the only optically inactive amino acid.