Answer:
The correct answer would be discovery of codons.
Marshall Nirenberg and Heinrich Matthaei experimentally decoded the triplet codons with the help of synthetic homopolymer of nucleic acid.
They used synthetic RNA which is made up of uracil units only that is, polyuridycil acid (poly-U) and a cellular component which could translate protein.
The poly-U RNA results in the formation of polypeptide chain entirely consisted of phenylalanine. Thus, UUU codes for phenylalanine.
Similarly, other codons were also deciphered.