Final answer:
Thomas Jefferson considered 3) England to be the most powerful European nation during his presidency, primarily because of Britain's dominance at sea and their policy of impressment that affected American merchant ships.
Step-by-step explanation:
During Thomas Jefferson's presidency, he considered England to be the most powerful European nation. Jefferson's administration faced significant challenges from both Britain and France, but it was the repeated harassment by the British Navy, through their policy of impressment, that clearly indicated British strength and dominance. Britain's control over the seas and its interference with American merchant ships were direct indicators of its power, leading to significant foreign pressures on the United States, including the eventual Embargo of 1807. Jefferson believed in applying economic pressure to address these challenges, which had some success but also led to domestic difficulties. The power struggle between Britain and France, as well as the European nations' conflicts, set the stage for the United States to establish its own path, influenced by the principles of the Enlightenment such as those from Rousseau and Montesquieu.