B

Swapping the last two residues of the B-chain (B29 and B30) produces little or no loss of biologic activity because the C-terminal segment B26–B30 is not critical for receptor binding; even complete deletion of B26–B30 retains near-normal potency [Nakagawa, 1986, PMID 3519607; Leyer, 1995, PMID 8567184]. By contrast, altering A-chain N-terminal residues (A1–A6) or breaking disulfide bonds would disrupt proper folding and receptor interaction, markedly impairing insulin function.