This is a strengthen with necessary premise that will require us to negate the correct answer choice and examine whether the negated form will weaken the argument.
The argument is saying that the "clean city" campaign is a success, despite what critics have said. This is backed up by the lower amounts of garbage on the streets today vs when we started the campaign.
In this case, they are assuming that the campaign is the cause of the lower trash in the streets.
This is what answer choice A is saying. If we negate this answer choice, it becomes "The amount of trash in city's streets was declining at the same rate or faster before the campaign than it did during the campaign". This destroys the argument because then how is the campaign responsible for declining street garbage if no campaign was as or more effective?
Answer choices B is incorrect because it discusses what the critics are aware or are unaware of. Negated this becomes, "the critics are aware..." but it doesn't matter what they are aware of. All that matters are the facts. And whether they have the correct interpretation of the facts is not clear in this answer choice.
I hope this helps. Please let me know if you have any other questions!