Until Antoine Lavoisier proved otherwise in the eighteenth

I think the beginning of the independent clause should start with Phlogiston, so that got me down to just C & D...I believe D is in passive voice, so naturally I am left with C....

In (A) : many scientists had believed that combustion released phlogiston, an imaginary substance whose properties were not fully understood

"an imaginary substance whose properties were not fully understood" is modifying phlogiston but sounds as if the bolded part of the sentence is a fact qualifying phlogistion without saying that it was a misbelief. I am not sure if I am clear but I hope you get my point.

Here is another example:

Many scientists believe that alcohol, an organic compound, is harmful to health.

Does the above sentence explicitly state that scientists believe alcohol to be an organic compound??? No. It only means that scientists believe alcohol to be harmful to health.

Similar is the case with (A). I am not convinced but I might be wrong. Any comments? I can post the OA but would like to bump the thread further for explanations.

Until Antoine Lavoisier proved otherwise in the eighteenth century, many scientists had believed that combustion released phlogiston, an imaginary substance whose properties were not fully understood.

(A) many scientists had believed that combustion released phlogiston, an imaginary substance whose properties were not fully understood
[Correct - use of HAD is correct because two things happened in past, one after another]

(B)many scientists believed that phlogiston was an imaginary substance released by combustion and its properties were not fully understood
[Incorrect - This construct states that scientists were misunderstood whether phlogiston was an imaginary substance but they were misunderstood whether combustion released phlogiston.]

(C)phlogiston was an imaginary substance whose properties were not fully understood and which many scientists had believed was released by combustion
[Incorrect - This construct states that Antoine Lavoisier proved phlogiston to be a real substance and what scientists believed is an additional information only.]

(D)phlogiston, an imaginary substance whose properties were not fully understood, was believed by scientists to be released by combustion
[Incorrect - had is a better choice instead of was and construct uses passive voice.]

(E)many scientists had believed that phlogiston was released by combustion and was an imaginary substance whose properties were not fully understood
[Incorrect - This construct uses passive voice and extra use of "and was" that could be replaced by a comma. Therefore A is preferred over E]

Please explain your answers.

Which is exactly why you would need the opening modifier Until Antoine Lavoisier proved otherwise in the eighteenth century. This modifier shows that the scientists belief was incorrect.

The second modifier is saying that phlogiston is an imaginary substance anyways. The sentence is about the wrong belief of scientists that combustion released phlogiston. This is correctly highlighted by the opening modifier.

OA is A.

gluon - Thanks. Ur explanation is very very nice.