Final answer:
The code for a single amino acid is provided by a sequence of 3 letters in an RNA molecule.
Step-by-step explanation:
The answer to your question is d. 4.
In genetics, the code for a single amino acid is provided by a sequence of 3 letters in an RNA molecule. These letters are called codons and each codon consists of 3 nucleotides.
Therefore, it takes 3 letters in sequence to code for a single amino acid.