Foods high in tryptophan, an amino acid that is a precursor to seroton

A list of three things is underlined in the original sentence, indicating the need to test
the sentence for parallelism. According to the non-underlined portion of the sentence,
the three verbs starting each item in the list indicate how the substance (tryptophan)
helps people to fall asleep: by "reducing", "relaxing" and "regulating." At first glance, it
appears that these 3 verbs - each ending in "ing" - are nicely parallel. However,
consider that the list is introduced by the preposition "by." It should read "by x, y, and
z" where x, y, and z are parallel. Instead, the list reads "by x, y, and in z." The use of
the word "in" to introduce the third element in the list throws off the parallelism and
creates a nonsensical phrase: "by . . .in regulating."
(A) This choice is incorrect because it repeats the original sentence.
(B) This choice does not correct the original parallelism error; it still uses "in" to
introduce the third item in the list. Moreover, that third item--"the regulation of"--is no
longer in the "ing" form, further distorting the parallel structure. Finally, the choice
creates a new problem. "By" is sufficient to introduce the three items in the list; "by
means of" is redundant.
(C) This choice correctly omits the word "in" when introducing the third item on the
list. However, the phrase "regulation of" is no longer parallel with the "ing" endings of
the first two items on the list.
(D) CORRECT. This choice corrects the original error by omitting the word "in" when
introducing the third item on the list. The three items now follow the parallel structure
"by x, y, and z."
(E) This choice corrects the original error by omitting the word "in" when introducing
the third item on the list. The three items now follow the parallel structure "by x, y, and
z." The choice creates a new problem, however. "By" is sufficient to introduce the
three items in the list; "by means of" is redundant.