The correct answer is A, as after the Adams-Onis Treaty, only the United States and Great Britain claimed the Oregon Territory.
The territory of Oregon was originally claimed by Great Britain, France, Russia and Spain; the Spanish claim was resumed after 1819 by the United States as a result of the Adams-Onis Treaty. The extension of the claimed area was imprecise at first, evolving over decades in the borders established in the 1818 treaty between British and Americans.
The United States based its claim, in part, on the entry of Robert Gray into the Columbia River in 1792 and the Lewis and Clark expedition into the area. Great Britain based its claims partly on British explorations of the Columbia River by David Thompson and prior exploration along the coast. The claim of Spain was based on the Inter caetera and Treaty of Tordesillas of 1493, as well as the exploration of the Pacific coast at the end of the 18th century. Russia based its pretensions on its explorations and commercial activities in the region and settled as its property the region north of the 51st parallel by the ukase of 1821, which was quickly overshadowed by the other colonial powers and retreated to the parallel 54 ° 40 'north by various treaties signed separately with the United States and Great Britain in 1824 and 1825, respectively.
Spain ceded its claims to the territory through the Nutka Conventions of the 1790s and the Adams-Onis treaty of 1819. In the Nutka Conventions, which followed the Nutka crisis, Spain granted Great Britain rights for access to the Pacific Northwest, although it did not establish the boundary with Spanish California, nor did Spanish rights cease in the area, Spain later renounced any remaining claims from the territory north of the 42nd parallel to the United States as part of of the Treaty of Adams-Onís of 1819. In the 1820s, Russia renounced its claims to the south of the parallel 54 ° 40' north and east of the 141 ° meridian in separate treaties with the United States and Great Britain.
From these treaties, the British and American presence became effective, being called this extensive region as "Oregon Country". The Americans soon extended their settlements and trading posts to the Pacific coast, in order to get a land route that would allow them to open their markets to Japan and China.
