A peptide chain is a polymer of amino acids linked together by peptide bonds. A peptide bond is an amide linkage formed between the carboxyl group ($-COOH$) of one amino acid and the amino group ($-NH_2$) of the adjacent amino acid, with the elimination of a water molecule . The repeating backbone of a peptide chain is represented generally as $-NH-CH(R)-CO-NH-CH(R')-CO-$. Since the options are image-based and not provided in the text, the correct option cannot be visually identified, but a typical peptide chain will show this $-CO-NH-$ repeating linkage.