Final answer:
The codon for phenylalanine is universally UUU in mRNA and TTT in DNA; thus, for dogs, it would also be UUU, which is option B.
Step-by-step explanation:
The question is asking for the triplet code for the amino acid phenylalanine in dogs, when given that the codon triplet AAA codes for phenylalanine in humans. The genetic code is universal, which means that the same codon codes for the same amino acid across all species. However, in the question, there is a mistake because the triplet AAA codes for lysine, while phenylalanine is coded by UUU in mRNA and therefore by TTT in DNA. Since the genetic code is universal, the codon for phenylalanine in dogs would be the same as in humans, which is UUU for mRNA and TTT for DNA. So the correct answer is B. UUU.