Which of the following are classified as stop codons?

Stop codons are specific nucleotide triplets within mRNA that signal the termination of protein synthesis during translation. The three recognized stop codons in the genetic code are UAA, UAG, and UGA. These codons do not encode for any amino acids and instead prompt the ribosome to release the newly synthesized polypeptide chain, effectively ending the process of translation.

The other choices involve codons that either initiate translation, encode for specific amino acids, or do not relate to stopping translation at all. For instance, AUG is known as the start codon, initiating the process of translation. The other sequences listed in the remaining options encode various amino acids, rather than functioning as stop signals. Therefore, only UAA, UAG, and UGA are classified as stop codons, making that answer the correct choice.