49th Parallel is the boundary line between which two countries?

The correct option is USA and Canada.

Detailed Explanation:
The 49th parallel north is a circle of latitude that lies 49 degrees north of the Earth's equator. It forms a major portion of the international boundary between the United States and Canada. This line of latitude separates the Canadian provinces of British Columbia, Alberta, Saskatchewan, and Manitoba from the US states of Washington, Idaho, Montana, North Dakota, and Minnesota.
The boundary along the 49th parallel was initially established by the Treaty of 1818 between the United States and Great Britain (which then controlled Canada) for the region spanning from the Lake of the Woods westward to the Rocky Mountains. It was later extended all the way to the Pacific Ocean by the Oregon Treaty in 1846.

Context for Other Options:
- North and South Vietnam: Historically separated by the 17th parallel north (established by the 1954 Geneva Accords) before the reunification of Vietnam in 1976.
- Germany and France: The border between these two nations is defined by geographic features such as the Rhine River and historical treaties, not a specific parallel line.
- Brazil and Chile: These two countries do not share a common border, as they are separated by Argentina and Bolivia.