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 - you can't say "much of the 'plural noun'"
(B) much of the currently uncontrolled dioxins - same problem
(C) much of the dioxins that are currently uncontrolled - same problem
(D) many of the dioxins that are currently uncontrolled and North Americans are exposed to come - there is no verb for the subject "much" within the relative clause
(E) many of the currently uncontrolled dioxins to which North Americans are exposed come - correct!

So, you could have gotten this one through process of elimination. But the reasons why E is right:
1. "many" matches with the plural noun "dioxins"
2. "to which" makes the rest of that clause one big modifier until the verb
3. the verb "come" is a plural verb to match with the plural subject "many"