It is widely believed that the most environmentally sensible thing to do is to buy food from local farmers whenever it is available.
Conclusion (so)
Sometimes it's environmentally preferable to buy non-local food.
Evidence
The distance food travels is only a small part of its environmental impact. Certain foods can be produced with far less impact in some places rather than others.
Evaluate
Whenever we're reading arguments on Role / Main Conclusion, we're anticipating rebuttals, since a majority of these arguments are rebuttals.
The paragraph opens with an attribution: the most environmentally sensible thing is to buy food locally.
We'd anticipate, from that, that our author is going to argue against that, and following the BUT pivot, we see that indeed she is pushing back against that idea.
It's most common on Main Conclusion / Role to have a disagreement conclusion like "But this is wrong" followed by evidence.
But in this case, the author pivots into premises and ends with the conclusion (marked by "So").
Goal
We're being asked about the attribution claim from the first sentence, which is an Opposing Idea. Our author's argument is designed to refute that. Her conclusion is essentially saying the opposite of that first idea.