In my opinion C

I going with E on this one.

Hmmm... I researched the origin of this question on GMATClub and it doesn't seem to have ever been posted with an official answer OR with a source. So I question the reliability of the question. There are clearly two conclusions and they don't really relate to one another. Those who assume that the latter conclusion is the "main point" as opposed to the former are in error, because the GMAT regularly puts conclusions at the beginning or middle of an Argument. Furthermore, each Argument is flawed; this would be a far more appropriate stimulus for a Strengthener or Weakener targeting one of the two parts of the Argument. As a result, I doubt the legitimacy of this question and I'm not sure it would pass muster on the GMAT.

That being said, You are correct, and E, if anything, is the best answer. The reason for this is that the Argument makes sense without the point about preventing dimentia. The first sentence tells us that women with heart attacks tend to develop dimentia; the third sentence tells us that women with heart attacks are also more common to suffer vision loss; the final Conclusion connects these elderly women with dimentia to those with vision loss. As a result, the intermediate conclusion is really a digression from this line of reasoning, supporting E. Choice D is completely wrong because the bolded statement has no effect on the other Conclusion in the Argument, and choice A can never be correct, because by definition assumptions are unstated and can never appear in the stimulus itself.

So, in conclusion, I agree with your answer choice, but I do not believe this issue will arise on test day because I do not think that this is a valid question as writen.