Final answer:
The correct answer to which amino acid translation is always initiated at is methionine, as the AUG codon serves as the start codon in the synthesis of proteins.
Step-by-step explanation:
Translation is always initiated at a codon for the amino acid methionine. The codon AUG serves two key roles in the process of translation: it codes for the amino acid methionine when it appears in the middle of a series of codons, and it functions as the start codon signaling the beginning of protein synthesis when it is located at the start of an mRNA sequence. Although the initial methionine is often enzymatically removed after the polypeptide chain starts to form, it is still the first amino acid incorporated during translation, making option D - Methionine the correct answer