Science writer: Studies indicate that North Americans are becoming more lethargic.
Conclusion (Thus)
The lethargy studies do not settle the question of whether fast food is unhealthy.
Evidence
Studies show North Americans are getting more lethargic, and from 1970 to 1990, North Americans had a 7x increase in fast food consumption.
(implied) But that correlation doesn't prove causality. Here's an alternate explanation for why lethargy has been growing: lack of exercise.
Few lethargic adults do much exercise, and lack of exercise can contribute to lethargy.
Evaluate
Over 85% of Main Conclusion and Role questions try to hide the conclusion by making it the very First Sentence, or by attaching it to some Pivot / Rebuttal.
The first sentence presents as a fact, not an opinion, so it doesn't seem like the Conclusion is our first sentence. The second sentence begins with an Attribution, "One researcher has concluded X".
We anticipate that our author will be resisting this conclusion. Waiting for that But / Yet / However pivot ... we get it with the second to last sentence.
But / Yet / However's are what we call the Author's Pivot. They indicate that we're pivoting to the author's argument.
The author might say But [conclusion]. After all, [premise]. Or the author might say But [premise]. Therefore, [conclusion].
In this case, we see Thus introduces the final claim, so that last claim is our Conclusion.
Goal
Once we've correctly located the Conclusion, we often want to rephrase it. If there are any pronouns, we import the idea they refer to. We try to think of a synonymous way of saying the same claim, because correct answers frequently go for a meaning-match, not a word-match.
The author is saying the lethargy studies don't settle the question of whether fast food is unhealthy. That could be rephrased as,
Let's look for the answer that best matches the meaning of the final sentence.