Answer:
France
Step-by-step explanation:
Based on this map, it appears that the nation that would have had the most difficulty enforcing a single state religion would be France. France has a relatively diverse religious landscape, with significant populations of Roman Catholics, Protestants, and Muslims, as well as smaller communities of Jews and Buddhists. In addition, France has a long history of secularism and separation of church and state, which would make it difficult to impose a single state religion. Other countries on the map with significant religious diversity include Germany, the United Kingdom, and Italy.