Step 1: Recall the structure of insulin.
Insulin is a small protein hormone secreted by the pancreas. It was the first protein to have its complete amino acid sequence determined.
Step 2: Identify the two polypeptide chains.
Insulin consists of two chains joined by disulphide bonds: chain A and chain B.
Step 3: Count amino acids in chain A.
Chain A contains 21 amino acids.
Step 4: Count amino acids in chain B.
Chain B contains 30 amino acids.
Step 5: Add them up.
\[ \text{Total} = 21 + 30 = 51 \text{ amino acids} \]
Step 6: Match with options.
The total is 51, matching option 3. \[ \boxed{51} \]