Step 1: Deciphering the genetic code required knowing exactly which three-base codon specified which amino acid, and this needed synthetic RNA or DNA sequences of precisely known, repeating composition to test in translation systems.
Step 2: Har Gobind Khurana pioneered organic chemical methods to build these defined polynucleotide sequences in the lab during the early 1960s, complementing the triplet-binding assay approach used by Nirenberg's group, and together this work completed the assignment of all 64 codons.
Step 3: Watson's fame rests on DNA structure discovery, McClintock's on jumping genes in maize, and Sanger's on sequencing methods, so none of them match the specific achievement of chemically synthesizing polynucleotides. The correct answer is Har Gobind Khurana.