Q.Among the following L-serine is

Step 1. Serine is one of the twenty standard amino acids, with the simple structural formula HOCH₂-CH(NH₂)-COOH -- an α-carbon bearing NH₂, COOH, H, and a short -CH₂OH side chain carrying the single defining hydroxyl group.

Step 2. Match each drawn option against known amino-acid side chains. Option (a), with a guanidino group (-NH-C(=NH)-NH₂) at the end of a propyl chain, is the side chain of arginine, not serine. Option (b), with a five-membered ring containing two nitrogens (an imidazole ring) attached via a -CH₂- linker, is the side chain of histidine, not serine. Option (d), with both a branched -OH and a -CH₃ group on the carbon adjace …

Identify each option's side chain (R group) and compare it against the known side chains of the twenty standard amino acids -- serine's side chain is the simplest hydroxyl-bearing one, -CH₂OH, distinguishing it from the bulkier guanidin …

• Confusing serine (-CH₂OH side chain) with threonine (-CH(OH)CH₃ side chain, option d) -- both carry a hydroxyl-bearing side chain, but threonine' …