Answer: E

The organization believes that by spicing up its newsletter, it will boost its popularity, counteracting the recent decline in membership. The underlying assumption is that one has anything to do with the other. If members are leaving the group for some other reason - say, a scandal concerning management of funds – spicing up the newsletter is unlikely to do any good.

Thus, in this strengthen question, we're looking for a choice that confirms the assumption. (E) is the only choice that does that, so it is correct.