shoule be D. i ruled out C because it treats overcome by pollution and routinely fell as two different clauses. Infact, both are related and should be under cause-effect relation.

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 (this refers to birds => birds promted)
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 (it refers to birds => birds promted + it singular vs birds plural)
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 (were promting...continuous...prompting happened over a period of time. Generally continuous is used to convey someother in background while some other action happening in foreground, eg, Boys were playing when the teacher arrived. In E - scenario is opposite...simple past used for background and continuous for foreground)

[spoiler]Now C vs D

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

'had been' - perfect continuous...but no simple past event exists + 'had been overcome...and fell => two separate event...when in fact, fact is the result of the first

D. birds overcome by pollution routinely fell from the sky above Los Angeles freeways, prompting officials in California to devise a plan to reduce (correctly addresses problems in C).
For me 'plan to reduce' and plan that reduced' both are correctly used. IMO - D[/spoiler]

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

D is the proper fit.

E uses a unnecessary "and". Usage of "were " does not refelect to "fall" .