Which amino acid's deficiency makes dietary choline essential for phosphatidylcholine synthesis?

I guess it isMethionine.

Methionine is the correct answer. 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. Tts activated form, s-adenosyl-l-methionine, is a very common methyl group donor.