Darla does not question any of Charles’s premises. She does not deny that unemployment rises during a recession, or that fewer people commute in cars to jobs. She just brings up an additional fact that makes the result of those premises seem less clear.

Darla’s claims do not contradict Charles’s conclusion. Her claims could still be true, and Charles's conclusion also be true. For instance, people might be using older cars in a recession, but because they use them so rarely, the overall effect could still be to reduce pollution from car exhaust. Darla's claim weakens Charles's argument given the current evidence, but it doesn't necessarily contradict it.

This is correct. The additional consideration is that during a recession fewer people buy new cars, and older cars tend to emit more pollutants. This additional information makes Charles's evidence seem less likely to lead to his conclusion, since we don't know if the trends he mentions or the trend Darla mentions will have more of an effect on pollution. We now need additional information to be able to determine the net effect of a recession on exhaust pollution.

Darla doesn't argue that Charles is right about air pollution from car exhaust decreasing. Though she doesn't say Charles is necessarily wrong, either, she at least questions his argument.

Darla’s claims are all about a consideration — an additional premise — Charles’s argument has overlooked. She doesn't claim that his premises themselves point to some conclusion that he has overlooked, let alone an absurd one.