The mayor plans to deactivate the city's fire alarm boxes

Hi prep,

I am happy to respond.

Conclusion of the above argument - some alarm boxes are still necessary.
Reason as suggested from the argument - the city’s commercial district, where there is the greatest risk of fire, has few residents and few public telephones. SO, this goes against the mayor's plan/claim. So, the alarm boxes will still be necessary for the city's commercial district.

Option C - The fire department gets less information from an alarm box than it does from a telephone call.
This option is stating the advantage of the telephone calls over alarm boxes. That's good. But as we already know from the argument that the city’s commercial district, where there is the greatest risk of fire, has few public telephones, so instead of the advantages of the telephone calls, we still need the alarm boxes. So, this option makes the argument stand as it is. Therefore, incorrect.

Step by Step approach -
Its a weaken question, so,

1. Firstly, look for the conclusion.
2. Look for the premises which makes that conclusion hold. (Then, try to predict an answer. That will solidify your reasoning for the argument.)
3. Look for an alternate cause or something that weakens the conclusion along with the reasoning that is leading that conclusion to hold.

If we look at the correct option for this question,

Option B - Commercial buildings have automatic fire alarm systems that are linked directly to the fire department.
This provides an alternate reason which weakens our conclusion of requiring the alram boxes still. So, correct.

Hope you got the soultion for your question.