Whose experiments cracked DNA and discovered unequivocally that a genetic code is a "triplet"?

Alfred D. Hershey and Martha Chase used radioactive phosphorus and radioactive sulfur and showed that DNA serves the genetic information for viral replication. 

Morgan and Sturtevant worked on Drosophila and gave chromosomal theory of heredity. 

Marshall Nirenberg and Heinrich Matthaei incubated synthetic polyuridylate, poly(U), GTP, ATP,  E. coli extract and a mixture of the 20 amino acids in 20 different tubes containing a different radioactively labelled amino acid. The poly(U) mRNA has many successive UUU triplets and thus, the resultant polypeptide should contain only the amino acid encoded by the triplet UUU. One out of 20 tubes synthesized a radioactive polypeptide that contained radioactive phenylalanine. It was concluded that the triplet codon UUU encodes phenylalanine. Correct answer is B.