The conclusion in the argument is: "to be healthy, it is sufficient to be slightly overweight."
Why? Because even though being overweight has been linked to a variety of health problems, recent research conclusively shows that people who are slightly overweight are healthier than those who are considerably underweight.
So, we are taking a relative property between those who are slightly overweight and those who are overweight, and concluding an absolute property based off of this relative property.
This is exactly what (E) is pointing out: "mistaking a merely relative property for one that is absolute."
See how the evidence is a comparison and yet the conclusion is an absolute?
For example, the above reasoning is similar to saying: smoking has been linked to health problems, but research shows that those who smoke only one cigarette a day are healthier than those who smoke a pack a day. Thus, it is sufficient to smoke only a cigarette a day in order to be healthy.
As you can see, the argument is using a property that is relative to conclude an absolute property.
Hope that clears things up! Please let us know if you have any other questions.
