Imperceptible to the eye or the ear, carbon monoxide is formed from

The answer is (E)
(A) this isn't a complete sentence, as "are only partially combusted" doesn't have a subject
(B) this isn't a complete sentence, as "are only partially combusted" doesn't have a subject
(C) this isn't a complete sentence, as "are only partially combusted" doesn't have a subject
"Imperceptible to the eye or the ear" wrongly modifies "the formation of carbon monoxide", it should modify carbon monoxide itself.
(D) this isn't a complete sentence, as "are only partially combusted" doesn't have a subject
(E) Correct.
"its" correctly modifies "carbon monoxide"
"Imperceptible to the eye or the ear" correctly modifies "carbon monoxide
"when materials ... are only partially combusted" completes the relative clause.

Answer is D not E as ( is formed ) is better than ( forms )

Yes, I totally agree. Thinking back, "its" is redundant in this sentence.

We need when to connect the phrase "are only partially combusted" to the sentence. A, B, C are out.
Between D and E, E has its as a pronoun which can refer to carbon monoxide. But that distorts the meaning of the sentence.
D is the winner.

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KAPLAN OFFICIAL EXPLANATION:

As we saw in number 17, this sentence begins with a modifier. Unlike the modifier in that sentence, this one is used correctly because it does indeed modify carbon monoxide. Eliminate (C) for creating an error where there was none. Next, you need to decide whether from, by, or when should end the underlined section. When you originally read the sentence, you probably noticed that something was very wrong. Since you can't change the end of the sentence, you need to alter the underlined portion in order to make the sentence grammatically correct. Only when does that (try reading them in), so eliminate (A) and (B). Finally, (D) is better than (E) because in this case the passive is formed is more accurate than forms and because the its in (E) is unnecessary. Go with (D).
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