Identify Amino Acid and Iodinated Hormone Pair

To identify the correct pair of the amino acid $(A)$ and the hormone $(B)$, let's analyze the single-letter codes and chemical structures:

1. Amino Acid Single-Letter Codes:

   • $\text{T}$ represents Threonine.
   • $\text{Y}$ represents Tyrosine.

2. Hormone Derivation:

   • Thyroxine ($T_4$ or tetraiodothyronine) is a thyroid hormone synthesized in the thyroid gland via the iodination of the aromatic ring of the amino acid Tyrosine ($\text{Y}$).
   • Insulin, on the other hand, is a peptide hormone consisting of 51 amino acid residues linked by disulfide bridges, rather than a derivative of a single amino acid.

Thus:

• Amino acid $(A) = \text{Y (Tyrosine)}$
• Hormone $(B) = \text{Thyroxine}$

Therefore, the correct option is C.