**Core Concept**
Human insulin is a hormone composed of two polypeptide chains, the A chain and the B chain, which are linked by disulfide bonds. The A chain contains 21 amino acid residues, while the B chain contains 30 amino acid residues.

**Why the Correct Answer is Right**
The A chain of insulin has an amino acid sequence of 21 residues, consisting of alanine (A1), phenylalanine (A2), and the remaining residues from A3 to A21. Similarly, the B chain has a sequence of 30 amino acid residues, including asparagine (B1), phenylalanine (B2), and the remaining residues from B3 to B30. The correct sequence and number of amino acids in both chains are crucial for understanding insulin's biological activity and its role in glucose regulation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option incorrectly states the number of amino acids in the A and B chains. The A chain actually contains 21 amino acids, not 30.
**Option B:** This option incorrectly states the number of amino acids in the B chain. The B chain actually contains 30 amino acids, not 32.
**Option C:** This option incorrectly swaps the number of amino acids in the A and B chains. The correct sequence is A (21 amino acids) and B (30 amino acids).

**Clinical Pearl / High-Yield Fact**
Recall that insulin is a heterodimeric protein composed of two polypeptide chains linked by disulfide bonds. Understanding the structure and composition of insulin is essential for appreciating its biological activity and its role in glucose regulation.

**✓ Correct Answer: D. 21, 30**