If this were true, then the data used in the study would seem unreliable, which would undermine the only evidence the author provides. This would weaken the author’s argument, whereas we’re trying to strengthen it.

This doesn't strengthen the conclusion. Knowing whether or not the surgeries were meant to address the abnormalities doesn't tell us anything about what caused the abnormalities, which is what we're interested in. We're trying to strengthen the claim that snoring can cause throat damage.

Knowing that the study was based on a sample of people who were all roughly in the same state of health might make the study seem more reliable. But we already assume the results of the study are reliable, since they are offered as one of the premises. We're interested in strengthening or weakening the conclusion, which interprets the results of the study as indicating that snoring causes throat damage. In fact, this answer choice potentially weakens the conclusion, since, if all these people were in a similar state of health, and they all came in for throat surgery, it's possible that they all had throat damage related to some shared health factor, like smoking or diet, rather than snoring.

The argument is about throat abnormalities in general, not just about who undergoes throat surgery. Though the author believes that snoring can damage the throat muscles, the author never says that these abnormalities were what brought people in for surgery in the first place. So throat surgery isn't a key part of our conclusion. It's only relevant because it was the source of the tissues used in the study. We're trying to strengthen the link between snoring and throat abnormalities, which this answer choice doesn't do.

This strengthens the conclusion by ruling out the potential counterargument that the causation could be the other way around. If we know for sure that these throat abnormalities don’t cause snoring, by eliminating that possibility, it seems somewhat more likely that the correlation between throat abnormalities and snoring actually comes from snoring causing damage to the throat.