Not necessary, because the author doesn’t argue that a doctor will probably be successful in getting someone to quit a habit by giving an exaggerated warning. The conclusion is that a doctor or other third party is necessary to use the strategy described; this doesn’t imply that a doctor will tend to be successful.

Not necessary, because the argument concerns a particular set of strategies involving exaggerating the dangers of a habit. This doesn’t imply that over half (”most”) of techniques can work for other habits.

Not necessary, because although exaggeration can make one more likely to quit, this doesn’t imply that the relationship between exaggeration and chances of quitting holds across the entire spectrum of exaggeration. It could be that there’s a certain threshold of exaggeration, above which it can make you more likely to quit. But if you go further above that threshold, it won’t necessarily increase the chances of quitting.

Necessary, because if it were not true — if people generally DO find it easy to deceive themselves — then we have no reason to think that a third party needs to be the one to use the deceptive exaggeration strategy. Individuals generally would be able to use the strategy without the need for others to deliver the exaggerated warning.

Not necessary, because whether something is “justified” is irrelevant. The author never argues that something is justified or not justified. The conclusion is a descriptive claim about the ease with which someone can use a particular strategy for quitting bad habits.