Insulin's Structural Features Steps:

• Insulin consists of two polypeptide chains (A and B), assembled via covalent links, defining its quaternary structure (statement 1 correct).
• Globular proteins like insulin bury hydrophobic R groups in the interior for stability, but insulin exposes some for receptor binding and dimerization in blood (statement 2 incorrect).
• The two chains are covalently linked by interchain disulfide bonds, termed sulfide bonds in this context (statement 3 correct).
• Thus, only statements 1 and 3 hold true.

Why C is correct:

• C identifies statements 1 and 3, aligning with the definition of quaternary structure (multiple chains) and covalent disulfide (sulfide) bonding in proteins.

Why the others are wrong:

• A includes 2, but hydrophobic R groups are not solely or primarily in insulin's center due to exposed patches for function.
• B includes 2 but omits 3, ignoring the key interchain bonding.
• D omits 1, overlooking insulin's multi-chain quaternary assembly.

Final answer: C