Conclusion: Revitalize business district ->increase No.of police
Premise: 20% reduce police -> decrease business revenue
Question: All weaken conclusion EXCEPT
(A) Two years ago, the city established more rigorous standards for the retention and hiring of its police officers. ->Explain the reason for 20% reduction of police official
(B) New businesses offering products or services similar to those in the district have emerged outside the district recently. -> Reason for decrease of district business revenue
(C) The number of people who reside in the district has not changed significantly over the last three years. -> Doesn't have any impact on Conclusion or Premise
(D) Businesses operating in the city but outside the district have experienced declining revenues during the last three years.-> Revenue reduction is a common phenomenal, not because of police official reduction.
(E) Some of the city’s police officers patrol areas outside as well as inside the district. -> So why only district business revenue went down why the police officers also patrol outside the district. Therefore, that revenue of biz in district went down dues to other reasons.

First I came up with A & C. Because A attacked the premise not the conclusion. But C doesn't help anything for conclusion or premise, I choose C as my final answer.