The boundary follows the 141st meridian west — option 4.

The border. Alaska belongs to the United States and lies north-west of Canada. For most of its length — the whole northern stretch, from the Beaufort Sea south to Mount Saint Elias — the boundary between Alaska and the Canadian Yukon runs precisely along the 141° W meridian, a straight line drawn on the map rather than any natural feature.

Why it is a straight line. Boundaries drawn along a parallel or a meridian are called geometrical or artificial boundaries. They are typical of regions that were mapped and divided by distant powers before being settled, and North America has several — the 49th parallel, which forms most of the Canada-United States border west of the Great Lakes, is the best known. Such lines take no account of terrain or of the people living across them, which is the standard criticism of them; the Alaska boundary, running through empty tundra and mountain, has caused less difficulty than most.

Some other well-known lines of this kind : the Radcliffe Line between India and Pakistan, the Durand Line between Afghanistan and Pakistan, the McMahon Line between India and China, the 38th parallel dividing Korea and the 17th parallel that once divided Vietnam.

Hence, the answer is 141° W.