The naturally occurring amino acid that contains only one basic functional group in its chemical structure is

Solution

Chemistry ma’am · 3 steps

Every amino acid already has its α-amino group, so look for the side chain that adds no second base.

1. The common base. All four have the α-NH₂ group next to COOH. That is one basic group each. The side chain decides whether there is a second one.
2. Amide is not basic. The lone pair on an amide nitrogen is delocalised onto the C=O group, so it is not free to accept a proton. An amide -CONH₂ therefore does not count as a basic group.

   Amino acid	Side chain	Basic groups
   Asparagine	-CH₂CONH₂ (amide, neutral)	1 (α-NH₂ only)
   Arginine	-(CH₂)₃NHC(=NH)NH₂ (guanidino)	2
   Histidine	-CH₂-imidazole ring	2
   Lysine	-(CH₂)₄NH₂ (amino)	2

3. So. Arginine, histidine and lysine are the basic amino acids, each with two basic groups. Asparagine has only one, so the answer is (a).

The idea to keep. To count basic groups in an amino acid, start with the α-NH₂ and then check the side chain. An amine, guanidino or imidazole adds one more base. An amide adds nothing.

Answer from the NTA final answer key. Solution by PrepWiser.