Birds falling

Whenever we see a construction with "and," such as "birds ____ and _____," we certainly want to look for parallelism.

In C, "... have been overcome and routinely fell" is definitely not parallel.

Be careful with A and E, though. The fact that "overcome" is a past participle and "falling" is a present participle does not necessarily mean that they can't be parallel. Consider the following:

After the game, the players were tired and bleeding.

This is a perfectly acceptable construction - both are acceptable descriptors of the players, and it wouldn't make sense to make them parallel. "... were tiring and bleeding" or "were tired and bled" both have different meanings. On the GMAT, it's ok to have past participles parallel to present participles, if the meaning makes sense. See Ron's explanation here: https://www.beatthegmat.com/most-brutal-sc-19-t9879.html

The larger problems with A and E are not problems of parallelism but of meaning.

In A, "this" is being used as a pronoun to refer to the entire idea of birds falling on freeways. The GMAT, generally speaking, does not like pronouns that stand in for general ideas, rather than specific antecedents. B can be eliminated for the same reason - "it" is general/ambiguous.

In E, if we remove all modifiers, we see that the core of the sentence is: "birds... were prompting officials." This doesn't make much sense.

Thus, the only answer choice remaining is D.