This does not affect the argument. We already know that the campaign had little impact on all people’s diets, including those who like vegetables. The author argues that it would be more successful if it targeted those who don’t like the taste of vegetables effectively.

This does not affect the argument. The campaign could just advertise the ways of making vegetables more appetizing that do not diminish their ability to help prevent cancer. “Some” could just mean that one method of preparation diminishes vegetables’ cancer-preventing abilities.

This does not affect the argument. There is no reason to suggest that liking a few vegetables would make you eat substantially more vegetables than someone who doesn’t like most vegetables.

This strengthens the argument. It provides evidence to believe that including information on how to make vegetables appetizing in the campaign would increase its effectiveness, as people would eat many more vegetables.

This weakens the argument by offering a very specific circumstance under which the campaign’s effectiveness would increase. The author doesn’t argue that vegetables must be appetizing, only that they should be made more appetizing (e.g., from horrible tasting to a little bad).