Internal acid base reaction of amino acids is due to formation of

Zwitter ion

The correct answer is Option (1) → Zwitter ion

Amino acids contain both a carboxylic acid group (–COOH, acidic) and an amino group (–NH₂, basic) in the same molecule. In neutral medium (or at isoelectric point), an internal acid-base reaction occurs where the carboxylic acid protonates the amino group, resulting in the formation of a zwitterion (dipolar ion): ⁺H₃N–CHR–COO⁻.

This zwitterionic form explains the high melting point, solubility in water, and amphoteric nature of amino acids.

• Lactam forms via intramolecular cyclization in γ- or δ-amino acids (not typical α-amino acids).
• Amide forms in acylation reactions.
• Peptide forms by intermolecular condensation between multiple amino acids.

Thus, the internal proton transfer uniquely leads to the zwitterion.