The correct answer is option 'A', Nirenberg and Matthaei.

Background:
The discovery of the genetic code, which determines how the nucleotide sequence of DNA is translated into the amino acid sequence of proteins, was a significant milestone in molecular biology. Understanding the genetic code is crucial because it provides insights into how genetic information is stored and transmitted.

Nirenberg and Matthaei's Experiment:
Nirenberg and Matthaei conducted a series of experiments in the early 1960s to decipher the genetic code and determine the relationship between nucleotide triplets (codons) and the amino acids they specify. They used synthetic RNA polymers that consisted of repeating units of a single nucleotide, uracil (U). These synthetic RNAs served as templates for protein synthesis in a cell-free system.

Experimental Procedure:
1. Nirenberg and Matthaei synthesized a synthetic RNA polymer containing only uracil (poly-U) and added it to a cell-free protein synthesis system.
2. They observed that the resulting protein was composed solely of the amino acid phenylalanine. This indicated that the codon UUU specifically codes for phenylalanine.
3. Next, they synthesized RNA polymers containing different combinations of uracil and cytosine (poly-UU, poly-UC, poly-UG, poly-UA) and added them to the cell-free system.
4. By systematically varying the nucleotide sequence of the synthetic RNA and analyzing the resulting proteins, they were able to determine the codons for different amino acids.
5. For example, they found that UUU and UUC both code for phenylalanine, while UUA and UUG code for leucine.
6. By extensively testing all possible combinations of nucleotide triplets, Nirenberg and Matthaei were able to crack the genetic code and establish that it is a triplet code, with each codon consisting of three nucleotides.

Significance of the Experiment:
Nirenberg and Matthaei's experiments provided the first direct evidence that a specific nucleotide sequence in RNA (later found to be DNA) specifies a particular amino acid during protein synthesis. This breakthrough led to a better understanding of the relationship between genes, RNA, and proteins. Their work formed the foundation for subsequent studies on the genetic code, which eventually led to the complete deciphering of the genetic code by scientists such as Marshall Nirenberg, Har Gobind Khorana, and Robert W. Holley.

Overall, Nirenberg and Matthaei's experiments were crucial in unraveling the triplet nature of the genetic code and significantly advanced our understanding of how genetic information is translated into proteins.