Choose a/an, the or no article to complete the following sentence. The river is in ____ north of the region.

The sentence requires an article to specify the location of the river. "The" is used before geographical features like north, south, east, west when they refer to a specific direction or part of a larger area. Therefore, "the" is the correct article to complete the sentence. The completed sentence is: "The river is in the north of the region."