The main subject of the sentence is what the sentence is primarily about. In this case, the sentence is discussing the hidden moral disputes in Yellowstone and how they provide wider lessons to America. Option A is incorrect because America is the recipient of the lessons, not the main subject. Option C is incorrect because the wider lessons are a result of the main subject, not the subject itself. Option D is incorrect because it is a context where ethical questions are tackled, not the main subject of the sentence.