Final answer:
The main idea of the article is that the Yukon River flows through subarctic land where few people live. Therefore, the correct option is D.
Step-by-step explanation:
The main idea of the article is stated in option D: The Yukon River flows through subarctic land where few people live. This sentence captures the central focus of the article by highlighting the geographical location and sparse population along the Yukon River. The article is likely to discuss the unique characteristics and challenges of this subarctic region.