Thanks for the question! So notice that this is a question that asks for a necessary assumption. So the question we have to ask ourselves about (E) is, is it a necessary assumption? Well, (E) tells us that the majority of people who successfully quit smoking cite health concerns as the primary motivation for their success. Does the argument require that a MAJORITY of people cite health concerns as the PRIMARY motivation for their success? Maybe slightly less than a majority cite health concerns as the primary motivation, whereas the rest of the people cited it as one of a few major concerns, but not technically the primary concern. Then the argument would still hold, and so we can see how (E) isn’t a necessary assumption based on the negation technique.
Hope this helps! Feel free to ask any other questions that you might have.