Final answer:
Guanidinoacetate is methylated using SAM to form creatine in the liver.
Step-by-step explanation:
The conversion of guanidinoacetate to creatine occurs in the liver.
Guanidinoacetate is methylated using S-adenosyl methionine (SAM) to form creatine. SAM is an important cofactor involved in methyl group transfers in the synthesis of amino acids and nucleotide metabolism.
The liver is the principal site of amino acid metabolism, and it plays a crucial role in various metabolic processes, including the breakdown of amino acids and the synthesis of creatine. Therefore, the correct answer is A) Liver.