Methionine-the correct answer is c. the key to answering this question correctly is an understanding that phosphatidylcholine is formed by donation of methyl groups. methionine is the only amino acid listed that can donate methyl groups. its activated form, s-adenosyl-l-methionine, is a very common methyl group donor.answer a is incorrect. asparagine is an essential amino acid with a negative charge. it can serve as a hydrogen ion recipient.answer b is incorrect. histidine is an essential amino acid with a positive charge. it can serve as a hydrogen ion donor.answer d is incorrect. threonine is an essential amino acid with an uncharged polar side chain. it contains a hydroxy group that can serve as a hydrogen ion donor or recipient.answer e is incorrect. valine is an essential amino acid with a hydrocarbon side chain; however, it is not a methyl group donor.