Final answer:
Three 'letters' or nucleotides of RNA make up a codon that codes for a single amino acid in protein synthesis.
Step-by-step explanation:
The how many 'letters' of an RNA molecule, in sequence, it takes to provide the code for a single amino acid is: C. 3.
An explanation for this is that in the genetic code, a sequence of three nucleotides, or 'letters,' in RNA composes a codon. Each codon corresponds to a specific amino acid or a stop signal during protein synthesis.
This is why three nucleotides are required to specify a single amino acid in the process called translation, where the sequence of the mRNA is used to build proteins. The universality of the genetic code among most organisms reflects this triplet nature, with some variations observed in mitochondrial DNA and certain other organisms.