Final answer:
A singlet presents three amino acids as possible codes.
Step-by-step explanation:
The genetic code works as a series of three-letter codes called triplets. Each triplet corresponds to one of the twenty common amino acids. There are a total of 64 possible combinations of nucleotide triplets, so a single amino acid can be encoded by more than one triplet. Therefore, the possible codes presented by a singlet are three amino acids (option A).