Which one of the following is the start codon?

Codons are the nucleotide base triplets that specify a particular amino acid in proteins. The start codon specifies the first codon of mRNA transcript that is translated. It codes for methionine in eukaryotes and a modified Met in prokaryotes. "AUG" is the most common start codon. AUG and UGA are start and stop codons respectively. Thus, the correct answer is option A.