The initial idea of triplet nature of codon was proposed by:

The correct answer is Option (1) → George Gamow

Option (1) George Gamow — Correct. The physicist George Gamow argued that 4 bases must code for 20 amino acids. Codons of two bases would give only 16, so he suggested the code is a triplet (4³ = 64 codons).

NCERT Extract:
Genetic Code – “… If determining the biochemical nature of genetic material and the structure of DNA was very exciting, the proposition and deciphering of genetic code were most challenging. In a very true sense, it required involvement of scientists from several disciplines – physicists, organic chemists, biochemists and geneticists. It was George Gamow, a physicist, who argued that since there are only 4 bases and if they have to code for 20 amino acids, the code should constitute a combination of bases.”