In the mid-1970’s, since birds were overcome by pollution,

Option A; The statement here means that addition to the pollution of the environment considered here occassioned by and perhaps, automobile emissions; habitual falling is also responsible for the littering of Los Angeles streets by birds. the comma (,) sign means that the statement continues with the (and) suggesting an inclusive corroborating addition.
Option A is wrong.

Option B; This infers that the birds that were previously surmounted were falling before now, and as of the time of filling this report which doesn't explain the current situation of things. Hence, option B is incorrect.

Option C; It suggests the action took place in the past. Birds had been overcome and the word fell means there is a nexus between the past participle 'had been' and the past tense 'fell'. It puts the examiner beyond doubts as to why a weaker influence would suffice for the most perfect fitting. Hence, it's a NO.
Option C is incorrect.

Option D ; Here, it's reported that 'birds having been tramped upon by pollution' fell habitually from the sky. This insinuates that the falling took place sometimes ago and it isn't currently happening.
The most correct option.

Option E; Continuous falling of the birds in addition to the pollution induced havoc means the two activities were jointly responsible for the prompt action embarked upon by the authority.
The option is wrong.