Step 1: Define stop codons.
Stop codons signal the end of protein synthesis. The three stop codons are UAA, UAG, and UGA.
Step 2: Identify the correct answer.
- UAA: A stop codon.
- UAG: A stop codon.
- UGA: A stop codon.
- AUG: A start codon, not a stop codon.
Step 3: Conclude.
AUG is the start codon, not a stop codon.
Final Answer:
\[
\boxed{\text{AUG}}
\]