Final answer:
The AUC and AUA codons both specifying isoleucine in mRNA is explained by the redundancy or degeneracy of the genetic code, where multiple codons can encode the same amino acid.
Step-by-step explanation:
The feature of the genetic code that explains why the AUC and AUA codons in mRNA both specify the amino acid isoleucine is known as redundancy or degeneracy. Redundancy in the genetic code means that more than one codon can code for the same amino acid. In fact, there are 64 possible codon combinations, but only 20 common amino acids are used to build proteins. This allows for some tolerance of mutations, as changing one nucleotide might result in the same amino acid being encoded, thus preventing possible detrimental effects on protein function.
The correct answer to the question is b. Degeneracy, which is another term for the redundancy found in the genetic code.