Q.Is (NH-CHR-CO)n a homopolymer or copolymer?

The unit $-(NH-CHR-CO)_n-$ is the general repeat unit of a polypeptide (protein), formed by condensation polymerisation of $\alpha$-amino acids, $H_2N-CHR-COOH$, with loss of water at each peptide (amide) linkage.

Although it is written with a single symbol '$R$' for compactness, in an actual protein the '$R$' group (side chain) is different for each amino acid residue along the chain (glycine has $R = H$, alanine has $R = CH_3$, and so on for the twenty or so naturally occurring amino acids). Since the chain is built from more than one type of monomer unit (many different amino acids), it is a copolymer. …

Recognise the -(NH-CHR-CO)- unit as the general peptide-bond repeat unit of a protein, then reason that the varying R group across the real chain …

• Assuming a single generic formula with one 'R' symbol must mean a single monomer (homopolymer) — missing that R varies residue-to-residue in an actual protein. …