Final answer:
A sequence of three nucleotide 'letters' in an RNA molecule, known as a codon, codes for a single amino acid.
Step-by-step explanation:
The student has asked how many 'letters' of an RNA molecule are needed to code for a single amino acid. The answer to this question is C. 3. In the genetic code, a sequence of three nucleotides in RNA, known as a codon, provides the code for one amino acid. This sequence of three is required because there are 20 standard amino acids but only four nucleotide bases in RNA (adenine, cytosine, guanine, and uracil). A combination of three nucleotides allows for 64 possible codons, which is more than enough to encode all 20 amino acids.