The author discusses moral issues, but ultimately sets them aside when discussing whether the surgery is effective, which is a question of fact. (A) isn't something that happens in the argument.

The author sets aside moral considerations when turning to the factual question of whether the surgery is effective at reducing recidivism. However, the author never states that only matters of fact are relevant to the overall issue—we don't know whether the author thinks the moral considerations are also important or not. (B) overstates what's going on in the argument.

Even if (B) were stated more precisely, it still wouldn't be a flaw. For the author's factual conclusion, it's true that factual evidence is what matters; moral issues don't make a difference to the factual effects of the surgery. Within the limited scope of the argument, it's not an flaw to set aside moral issues.

The author does discuss some of the issues that make the program controversial, when talking about how the program is "obviously morally questionable." (C) doesn't accurately describe the argument. And even if the author didn't discuss the moral issues, the program's controversy is part of the context, and not important to the core argument about the effects of the surgery.

The author never makes this assertion, so (D) can't be the flaw. The author discusses one question related to rehabilitation—whether the surgery is rehabilitative—without reference to morality, but that's not the same as saying that morality isn't relevant at all.

The author relies on evidence drawn only from the sample of prisoners who were offered the surgery—in other words, prisoners who behaved especially well in prison. And that's a reason to think that the sample is unrepresentative: their good behavior in prison is probably related to their conduct after being released. And this gives us reason to doubt the conclusion, because the surgery may not be the true cause of these prisoners' low recidivism.