What is traditionally known as the world's longest undefended border?

Discover Canada records this in one direct sentence. The guide writes: Millions of Canadians and Americans cross every year and in safety what is traditionally known as "the world's longest undefended border". The border the test wants is therefore the border between Canada and the U.S.A.

The phrase is a fixed expression. Discover Canada uses the words "the world's longest undefended border" in quotation marks, signalling that this is the traditional way of describing the Canada-U.S.A. boundary. The same passage notes that "millions of Canadians and Americans cross every year and in safety," grounding the phrase in real cross-border traffic between the two countries.

The Canada-U.S.A. relationship is rooted in geography. Discover Canada writes that "along the southern edge of Canada lies the Canada-United States boundary," and that "both Canada and the U.S.A. are committed to a safe, secure and efficient frontier." Elsewhere the guide describes "integrated Canada-U.S.A. supply chains" that connect the two economies. So the boundary is not just open — it is also a working economic line where supply chains flow back and forth.

The Peace Arch symbolises the relationship. Discover Canada writes that "at Blaine in the State of Washington, the Peace Arch, inscribed with the words 'children of a common mother' and 'brethren dwelling together in unity,' symbolizes our close ties and common interests." So the longest undefended border in the world is also marked by a monument framing the two countries as siblings — and the test answer rests on that traditional phrasing about the Canada-U.S.A. border.

The question is testing whether new citizens know which border Discover Canada describes as "the world's longest undefended border." The guide commits to one border: the Canada-U.S.A. boundary. The right test answer matches that.

The wrong answer choices each pick a different boundary. Canada does not border Russia or Mexico — only the United States across the southern edge. The U.S.-Mexico border is a separate boundary not described in Discover Canada. Only the Canada-U.S.A. border carries the traditional "longest undefended border" phrase.