The only optically inactive amino acid is

The correct answer is Option (1) → Glycine

All α-amino acids have the general formula \( \ce{H2N-CH(R)-COOH} \). The α-carbon carries four groups: –\( \ce{NH2} \), –COOH, –H and the side chain R. A carbon with four different groups is chiral, which makes the amino acid optically active.

The side chains of the given amino acids are:

• Glycine: R = –H

• Proline: the side chain –\( \ce{CH2CH2CH2} \)– joins back to the α-nitrogen, closing a five-membered ring

• Serine: R = –\( \ce{CH2OH} \)

• Lysine: R = –\( \ce{(CH2)4NH2} \)

In glycine, \( \ce{H2N-CH2-COOH} \), the α-carbon carries two hydrogen atoms, so it is not chiral. Glycine is the only common α-amino acid that is optically inactive.

Why the other options are incorrect

• Option (2) Proline — Incorrect. Its α-carbon carries –H, –COOH, the ring nitrogen and a ring \( \ce{CH2} \) group. These four groups are different, so the α-carbon is chiral and proline is optically active.

• Option (3) Serine — Incorrect. The –\( \ce{CH2OH} \) side chain makes the α-carbon chiral.

• Option (4) Lysine — Incorrect. The –\( \ce{(CH2)4NH2} \) side chain makes the α-carbon chiral, so lysine is optically active.

NCERT (2026-27 edition):
Classification of Amino Acids – …

Note: The same/similar question has been asked in different years by NTA as per the details given below: