Which one of the following compounds exists as a dipolar ion?

Amino acid

The answer is (2) amino acid.

Amino acids exist as dipolar ions, also known as zwitterions, because they have both a positively charged amino group and a negatively charged carboxyl group. This allows them to dissolve in both polar and nonpolar solvents.

Carbohydrates, long chain fatty acids, and α-halo carbonyl compounds do not have both a positively and negatively charged group, so they do not exist as dipolar ions.