Q.The most basic amino acid is

Step 1. Establish what 'most basic' means here. The basicity of an amino acid's side chain depends on how strongly its R-group can hold a positive charge (i.e., how high its side-chain pKa is) at physiological pH - the higher the pKa, the more strongly and completely the group stays protonated (positively charged).

Step 2. Check option (b) Histidine. Histidine's imidazole side chain has a pKa of only about 6.0, so it is only weakly and partially protonated at physiological pH (~7) - it is basic, but only mildly so.

Step 3. Check option (c) Glycine and option (d) Glutamine. Glycine's side chain is a single hydrogen atom with no ionisable group at all, so it contributes no basicity whatsoever. Glutamine's side chain is a neutral amide group, which is also not ionisable/basic under normal physiological conditions.

Step 4. Check option (a) Arginine. Arginine's side chain carries a guanidinium group with a pKa of about 12.5, the highest of any standard amino acid, so it remains fully protonated (positively charged) across almost the entire physiological pH range - making it clearly the most basic of the four options.

Compare the side-chain pKa / ionisable-group strength of each option.

• Confusing histidine (weakly basic, pKa ~6) with arginine (strongly basic, pKa ~12.5) since both are commonly listed as 'basic amino acids'.
• Forgetting that glycine and glutamine have no basic side chain at all.