I have 2 multiple questions as attached. I have done those but have some doubts. The questions are in red and my answer is in black.
Need to get explanation why the option B in #61 is wrong (heavy references) and why option A in #53 is wrong (also references)
Basically I need to know why only 1 option is good for each question.
For the first question, the correct answer is alanine.
Gln doesn't have a free amino group. It has an amide group.
Arg doesn't have a free amino group either. It has a very complex functional group.
Ser has no NH2 group at all. The only NH2 group it has will be bound up in the peptide linkage.