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The triplet codon, a three-nucleotide sequence, is used to describe each amino acid. Scientists hypothesised that single amino acids must be represented by combinations of nucleotides based on the fact that the "alphabets" of mRNA and proteins had different numbers of "letters."

The explanation for the correct option (D) degeneracy

Scientists hypothesised that the genetic code was "degenerate" and that amino acids were encoded by nucleotide triplets. In other words, many nucleotide triplets may encode the same amino acid. Here, in this case, the given codes are degeneric in nature. So, option D i.e., Degeneracy is the correct answer.

The explanation for incorrect options (A, B and C)

Since the given two codons code for the same amino acid isoleucine, it is degeneric in nature.

• Anticodons are nucleotide sequences that complement codons. They are present in tRNAs and enable the tRNAs to align the proper amino acid with an mRNA during the synthesis of proteins. It is not a complementary codon.
• UAG (amber), UAA (ochre), and UAG are the three nonsense codons (opal). a change in which a codon for chain termination is substituted with a codon for an amino acid (UAG, UAA, or UGA).
• There is a universal genetic code. With a few notable exceptions, almost all species synthesise proteins using the same genetic code. Here, it is only about one organism.

So, the rest of the options A.B and C are irrelevant to the question.