One thing to think about here is that the correct answer MUST have to do with evening hours and time of sunset, based on the stimulus. In fact only one of the answer choices [spoiler](A)[/spoiler] does that. But to more thoroughly understand why [spoiler](A)[/spoiler] really is a logical completion of the passage, think about the fact that we're told that if daylight savings time were extended, the sun would "continue to set an hour later during fall months." That means that for instance, 8 o'clock would be the time of sunset on a day on which under the current (non-extended-DST) system, the sun would set at 7 o'clock. If the stores are open till 8 (for instance), and the sun has set at 7, those people who tend to stay away from these stores after dark will not shop there during the stores' last hour of business. But if the stores stay open till 8 and DST has been extended so that it is also not until 8 that the sun sets, it will be light all the way until the stores close, so even those people who stay away from these stores after dark would continue to shop at the stores during all opening hours.

daylight saving is done to gain extra time that's it.
A suits the bill.