Each triplet codon codes for a specific amino acid. There are 64 triplet codons that code for 20 amino acids. Example: AUG codes for methionine, UGG codes for tryptophan, CCC for proline, and so on.
Similarly, in an artificial mRNA poly U chain, UUU coded for phenylalanine.
Triplet codons like UAA, UGA, and UAG are called stop codons that stop the process of translation.
So, the correct option is 'Triplet UUU code for phenylalanine'.