A public-service advertisement advises that people who have

The advertisement advises that people who have consumed alcohol shouldn't drive until it is safe to do so (which seems like reasonable advice).

However, in a study that took place in a hospital, people UNDERESTIMATED the time required required to regain their ability to drive after drinking. Here's an example: the subjects of the study are given five Jager bombs (sign me up!). They sit around until they think they can drive safely, and on average wait 4 hours before they say they are sober enough to drive. In reality, say it takes 6 hours for the Jager bombs to wear off and for the subjects to regain their ability to drive safely. These people have underestimated the time required between drinking and driving safely.

Based on this study, the author concludes that "many people who drink before driving will have difficulty following the advertisement's advice," because people are not great at judging whether they have waited long enough between drinking and driving. We are trying to support this conclusion with the correct answer choice.

Take a look at (B):

The subjects in the hospital setting ALREADY underestimated the time required between drinking and driving. A tells us that these subjects were actually MORE CONSERVATIVE (or more cautious) than people outside of a hospital setting.

To go back to our example, the subjects in the study estimated that they could drive after 4 hours, when they really needed 6. Let's say that people out at a bar on a Saturday night drink the same 5 Jager bombs -- if they are LESS CONSERVATIVE than the subjects in the study, maybe they think they can drive after only 2 hours.

This confirms that people are terrible at judging when they have let enough time pass between drinking and driving. In fact, it shows that people are even worse at making this decision than the study showed. By showing that people outside of the hospital underestimate the time needed to sober up, (B) supports the conclusion that "many people who drink before driving will have difficulty following the advertisement's advice" to wait between drinking and driving.

I hope that helps!