The correct answer is the following.
The reason that made the British relinquish forts in the Northwest and improve trade relations was the signing of Jay's Treaty.
Jay's Treaty was signed in 1795 by Britain and the United States in an attempt to avoid war. After some issues that could not be resolved in the Treaty of Paris after the Revolutionary War, both countries signed Jay's Treaty to maintain peace and support trade activities. Secretary of the Treasury Alexander Hamilton designed the treaty and received total support of President George Washington who sent John Jay to negotiate the terms with the British.