Final answer:
The correct answer is three. Three "letters" of an RNA molecule, in sequence, are needed to provide the code for a single amino acid.
Step-by-step explanation:
The correct answer is c) three.
Three "letters" of an RNA molecule, in sequence, are needed to provide the code for a single amino acid. These sequences are called codons, and each codon consists of three nitrogenous bases. The sequence of codons determines the sequence of amino acids in a protein.