NOT SURE ABOUT OA ..
but according to me ANS should be A

in A I am not comfortable with the word ENTIRELY.
I would go for C.

Is it B?

wats OA??
I am confused between B and E... The passage neither talks about increase of species in area nor about air quality....
I would go with B.

IMO: A
use the negation strategy

In most major cities, air-pollution problems are not caused almost entirely by local industry.
is this weaken the argument? NO.. then this can not be assumption
B

IMO: B

The option B is an inference from the statements in question.

I had like to wait for an expert's take on this

Went from eliminating easy to tough --


Option D is clearly an assumption, otherwise why would author advocate such steps?

Option E is also an assumption made here. The author is clearly using the number of birds in and around London as actual increase in the number of species

Option C -- is also an assumption. Negate this information and the argument falls apart. Clearly, the author is assuming that the problem is same everywhere

Out of A and B - The word "entirely" is bit extreme and I don't have any other reason to eliminate option B and select this option as the answer. There can be other problems as well but the air pollution could be the largest contributor to this problem. Therefore, we cannot use the word entirely here.

Correct answer OPTION A

In the years since the city of London imposed strict air-pollution regulations on local industry, the number of bird species seen in and around London has increased dramatically. Similar air-pollution rules should be imposed in other major cities. Each of the following is an assumption made in the argument above EXCEPT:

(A) In most major cities, air-pollution problems are caused almost entirely by local industry.
This is an assumption beacuse the city of London imposed strict air-pollution regulations on local industry only.
(B) Air-pollution regulations on industry have a significant impact on the quality of the air.
This is an assumption beacuse the city of London imposed strict air-pollution regulations on local industry resulting in increase number of bird species.
(C) The air-pollution problems of other major cities are basically similar to those once suffered by London.
This must be a valid assumption beacuse if this is not the case then there is no need to impose the same law in other city.
(D) An increase in the number of bird species in and around a city is desirable.
This is not the assumption beacuse the law was not imposed to increase the number of bird species. It was to increase the quality of air. This was an indirect result happened from the increase in the quality of air.
(E) The increased sightings of bird species in and around London reflect an actual increase in the number of species in the area.
This is an assumption made in the stated paragraph.

Therfore the anwere to this question IMO is D.