What is the smallest structural unit (monomer) of a protein called?

Given below are two statements:
Statement $I:$ Amino acids have a property of ionizable nature of $-NH_2$ and $-COOH$ groups,hence have different structures at different $pH$.
Statement $II:$ Amino acids can exist as Zwitterionic form at acidic and basic $pH$.
In the light of the above statements,choose the most appropriate answer from the options given below:

Which one of the following statements is wrong?

Which of the following amino acids contains a polar and uncharged $R$-group?

Which protein is present in the exoskeleton?

For growth and repair in the body,$.......$ are required.