When Britain agreed to let the 49th parallel be US/British boundary, what other state was being debated with Britain

Oregon Country

Explanation:

When Britain agreed to let the 49th parallel by the US/British boundary, the other state that was being debated with Britain is "Oregon Country"

This is evident in that, the 49th parallel agreement between Great Britain and the United States was about the north boundary of Louisiana Territory that lies between Lake of the Woods and the Rocky Mountains.

However, during this period, the two nations also decided to joint occupation of the Oregon country for some years.

just like you man, i personally think its either c or d

explanation:

but good luck with the test man