This SC has been discussed thoroughly on almost all the forums. : )
Do you have any specific queries that you feel have not been addressed in any of those discussions?
: )


There is no thrill for tutors to repackage the same information.

In the mid-1970's, since birds were overcome by pollution, and routinely falling from the sky above Los Angeles freeways, this prompted officials in California to devise a plan that reduced automobile emissions.

A. since birds were overcome by pollution, and routinely falling from the sky above Los Angeles freeways, this prompted officials in California to devise a plan that reduced
B. since birds that had been overcome by pollution were routinely falling from the sky above Los Angeles freeways, it prompted officials in California to devise a plan that would reduce
C. birds had been overcome by pollution and routinely fell from the sky above Los Angeles freeways, prompting officials in California to devise a plan that reduced
D. birds overcome by pollution routinely fell from the sky above Los Angeles freeways, prompting officials in California to devise a plan to reduce
E. birds overcome by pollution and routinely falling from the sky above Los Angeles freeways were prompting officials in California to devise a plan to reduce

Infinitive vs that clause is a genuine split and an important one in GMAT SC.
I have talked about it here: https://www.beatthegmat.com/2015-og-sc-68-t290211.html

Here, grammatically both are correct.
However, logically only infinitive is.

A,B,and C say that plan itself is performing the action of reducing emissions.
However, logic dictates that by using plan, emmisions can be reduced (as is communicated by D and E).
i.e,
Plan is not the subject( actual performer) of verb( reduced ).
Plan, however, is a mean to reduce emissions.

In the mid-1970's, since birds were overcome by pollution, and routinely falling from the sky above Los Angeles freeways, this prompted officials in California to devise a plan that reduced automobile emissions.

A. since birds were overcome by pollution, and routinely falling from the sky above Los Angeles freeways, this prompted officials in California to devise a plan that reduced
B. since birds that had been overcome by pollution were routinely falling from the sky above Los Angeles freeways, it prompted officials in California to devise a plan that would reduce
C. birds had been overcome by pollution and routinely fell from the sky above Los Angeles freeways, prompting officials in California to devise a plan that reduced
D. birds overcome by pollution routinely fell from the sky above Los Angeles freeways, prompting officials in California to devise a plan to reduce
E. birds overcome by pollution and routinely falling from the sky above Los Angeles freeways were prompting officials in California to devise a plan to reduce

Parallelism has two aspects:
One deals with grammer or structure,
while other deals with logic.

Structural parallelism is fine in E[1];
logical parallelism,however, is not.

An adverb such as consequently, therefore, or, thus, would have been much better to establish relationship between these two structually parallel constructions. In absence of such an adverb, it is all mutually exclusive.

On the contrary please note the construction in OA:

D. birds overcome by pollution routinely fell from the sky above Los Angeles freeways, prompting officials in California to devise a plan to reduce

red modifier is past participle phrase.
It modifies birds.
which birds?
the ones overcome by pollution.
Pariciple phrases do not modify in void: they also need to have a close relationship with clause(or verb).
In OA, we do have a close necessary relationship between pariciple phrase(overcome by pollution) and verb( fell).

PS:
[1] ved can be made parallel with ving;
i.e, past participle can be made parallel with present participle( just as in verb and verb ll-ism, tense can be different, if context demands)and we do have such instance, in which past participle is made llel to present one, in official material's correct answer choices.

In the mid-1970's, since birds were overcome by pollution, and routinely falling from the sky above Los Angeles freeways, this prompted officials in California to devise a plan that reduced automobile emissions.

A. since birds were overcome by pollution, and routinely falling from the sky above Los Angeles freeways, this prompted officials in California to devise a plan that reduced
B. since birds that had been overcome by pollution were routinely falling from the sky above Los Angeles freeways, it prompted officials in California to devise a plan that would reduce
C. birds had been overcome by pollution and routinely fell from the sky above Los Angeles freeways, prompting officials in California to devise a plan that reduced
D. birds overcome by pollution routinely fell from the sky above Los Angeles freeways, prompting officials in California to devise a plan to reduce
E. birds overcome by pollution and routinely falling from the sky above Los Angeles freeways were prompting officials in California to devise a plan to reduce

OA:D

In A and C, birds without modifier(s) attached to it is problematic.

In the mid-1970's, since birds were overcome by pollution, and routinely falling from the sky above Los Angeles freeways, this prompted officials in California to devise a plan that reduced automobile emissions.

A. since birds were overcome by pollution, and routinely falling from the sky above Los Angeles freeways, this prompted officials in California to devise a plan that reduced
B. since birds that had been overcome by pollution were routinely falling from the sky above Los Angeles freeways, it prompted officials in California to devise a plan that would reduce
C. birds had been overcome by pollution and routinely fell from the sky above Los Angeles freeways, prompting officials in California to devise a plan that reduced
D. birds overcome by pollution routinely fell from the sky above Los Angeles freeways, prompting officials in California to devise a plan to reduce
E. birds overcome by pollution and routinely falling from the sky above Los Angeles freeways were prompting officials in California to devise a plan to reduce

OA:D

In A and B, this and it, respectively, tries to refer to the preceding clause(read verb).
this and it cannot do this job.
,verbing , on the other hand,is perfect for this purpose.