I guess .. there is an issue in the underlining of the question. may you please edit the ques ..
Posting the correct question.

A report by the American Academy for the Advancement of Science has concluded that much of the currently uncontrolled dioxins to which North Americans are exposed comes from the incineration of wastes.


(A) much of the currently uncontrolled dioxins to which North Americans are exposed comes

(B) much of the currently uncontrolled dioxins that North Americans are exposed to come

(C) much of the dioxins that are currently uncontrolled and that North Americans are exposed to comes

(D) many of the dioxins that are currently uncontrolled and North Americans are exposed to come

(E) many of the currently uncontrolled dioxins to which North Americans are exposed come

dioxins is used as countable noun here ..

many is used for countable nouns. much is used for uncountable nouns
Hence we can eliminate A , B , C ... Between D & E, D is has parallelism error .. .Also it's wordy .

E is the better choice and is my answer .