Not necessary, because the author’s reasoning doesn’t require the specific risks of a sedentary lifestyle to be equal to the risks of smoking. What matters is whether the strategy of emphasizing negatives can work with pro-exercise efforts just as it did with anti-smoking efforts. A strategy of emphasizing dangers can motivate people to take action even if the exact risk level of those dangers has slight differences from activity to activity.

Not necessary, because the issue with the current efforts is that they’re not successful at getting people to exercise regularly. They may be successful at getting people to understand that there are positive benefits; but even if this is true, it still hasn’t translated to significantly more people exercising regularly.

Not necessary, because even most smokers are also aware of the positive benefits of quitting, it’s still the case that the anti-smoking efforts have focused more on the dangers of smoking, and these efforts have been successful. What matters is not people’s knowledge of benefits/dangers, but the nature of the anti-smoking efforts — did they focus on benefits or on dangers?

Necessary, because if it were not true — if anti-smoking efforts WOULD be more successful even if they emphasized the benefits of quitting rather than the dangers — then the anti-smoking efforts would no longer establish that focusing on dangers would be more successful. The negation of (D) indicates that focusing on benefits rather than dangers would actually be MORE successful in for anti-smoking efforts; so the anti-smoking efforts would no longer help justify a conclusion that focusing on dangers would be better in the pro-exercise context.

Not necessary, because the argument is based on the success of anti-smoking efforts, not on the internal motivations or perceptions of people who have quit smoking. So even if less than most people who quit smoking cite health concerns as the primary motivation, that doesn’t change the fact that there is evidence that a focus on dangers in anti-smoking efforts has been successful. The author can believe that this supports a conclusion about a focus on dangers for pro-exercise efforts without having any opinion about what proportion of people say they quit because of health reasons.