Answer: B) A subject.
Explanation: A noun clause is a dependent clause that acts as a noun. They can begin with words such as how, that, what, whatever, when, where, whether, which, whichever, who, whoever, whom, whomever, and why. And can act as subjects, direct objects, indirect objects, predicate nominatives, or objects of a preposition. In the given sentence, the noun clause is "whoever travels on the Yukon River in winter" and it is acting as the subject of the sentence.
