What is a one letter code of Tryptophan?

W

The correct answer is Option (4) → W

Amino Acid Codes

Amino acids, the building blocks of proteins, are commonly represented by both a three-letter code and a single-letter code for brevity in sequence analysis.

The choice of $\mathbf{W}$ for Tryptophan is mnemonic; its side chain contains a double ring structure, which visually suggests the two overlapping Vs that form a W.