First let's address the task. The question is just asking for any strategy that is used--it doesn't have to be the main point, the most-mentioned word, or the only strategy used. So we can't go for D just because exercise is mentioned a great deal and the similarity between the brain and muscles is not. We have to look at exactly what each answer is saying the argument does.
E says that the argument IMPLIES (not states) a similarity between brains and muscle. That's certainly the reasoning the argument is going with: Physical organs improve with exercise, and since the brain is a physical organ, it can improve with exercise. So you should buy this product to exercise your brain. Not a classic argument, really.

But they're relying on a similarity between the brain and other body parts. I agree that the "muscle" part of this is a bit odd--this could have been better-written--but it is one of the things that is getting compared, even if the statement is not very direct.
D, on the other hand, says that a careful analysis is made, and we just don't have that. The argument doesn't define exercise in any way, nor does it explain how it produces its results. It only tells us that exercise has benefits. That does not constitute a careful analysis. Imagine that you hadn't heard of exercise before. Would this have cleared it up for you? Not at all. You could only guess that it's something you do for your body, and that would be about it. Take the phrasing of the answer as literally as you can; where is the analysis?
ManifestDreamMBA
I had a similar reasoning and picked D too. Now I am scratching my head on when to give leeway to some words in the argument and when not to (here, muscle being physical organ for example. we cannot say brain and muscle have exercise in common, because that's what the argument is trying to get to)
Experts please help on why D isn't correct
MartyMurray DmitryFarber GMATNinjaAle300503
Nowhere in the argument is mentioned that brain and muscles are similar, since it is not even said that muscle is a physical organ.
Muscles are barely mentioned in the passage, and they are referred to as 'muscle tone', not even directly as 'muscles'.
The only thing that they have in common is that apparently both MUSCLE TONE and brain can be improved through exercise:
" Exercise leads to better performance of such physical organs as the heart and the lungs, as well as to improvement in muscle tone."
And then "Subscribe to Stimulus: read the magazine that exercises your brain."
So IMO (D) seems quite reasonable here:
(D) It supports its recommendation by a careful analysis of the concept of exercise.