Once made exclusively from the wool of sheep that roam the Isle of Lewis and Harris off the coast of Scotland, Harris tweed is now made only with wools that are imported, sometimes from the mainland and sometimes they come--as a result of a 1996 amendment to the Harris Tweed Act--from outside Scotland.

(A) sometimes from the mainland and sometimes they come
(B) sometimes from the mainland and sometimes
(C) that come sometimes from the mainland or sometimes
(D) from the mainland sometimes, or sometimes it comes
(E) from the mainland sometimes, or sometimes coming

The answer is B. I picked B as well when I took the test.
However, when I looked at the question again, I think that A is correct because "as a result of [noun phrase]" is a modifier that modifies an action or a clause.
If I put B in, the sentence becomes:
Once made exclusively from the wool of sheep that roam the Isle of Lewis and Harris off the coast of Scotland, Harris tweed is now made only with wools that are imported, sometimes from the mainland and sometimes--as a result of a 1996 amendment to the Harris Tweed Act--from outside Scotland.
I don't know what action is "as a result of..." modifying.
But if I put A into the sentence, it is clear that "as a result of..." modifies the action "come". Although A breaks the parallelism, I thought that grammatically correct is more important?

Being gramatically correct is important (of course) and parallelism is a very important component of "correct grammar."

Try reading the sentence without the information in between the dashes. In that instance, it becomes clear that B is the clearest, gramatically correct, and concise answer choice.

When you see information in between dashes -- ... ... ... -- think of that information as more of a 'pause' in the sentence versus direct modification as you would see around commas, etc.

Hope that helps.

-Brian Lange
MGMAT

I am not seeing why C is wrong. Can anyone help!?

"that" in C can't refer to anything.
logically (with the intended, correct meaning), the "that" refers to the imported wools; but "wools" is plural, and "that" can't refer to a plural.

If you try to think that the "that" parallels with the preceding "that", then you need an "and". Otherwise, it would be a run-on sentence.

you can't follow 'that are imported' with 'that come'; that's 2 main verbs in one clause, which is a big no-no unless those verbs are joined by 'and'.

notice the idiomatic usage: sometimes X and sometimes Y is the correct form when X and Y are alternative options.