Which of the following amino acid plays an essential role in the DNA methylation process?

DNA methylation is catalyzed by DNA methyltransferases (DNMTs) that transfer a methyl group from S-adenosyl methionine to the Carbon at 5 positions in the Cytosine (5C) ring. It is also denoted as 5methylCytosine or 5mC. The methyl donor S-adenosyl methionine is a derivative of the amino acid Methionine.