I believe the answer is A the Atlas Mountains
Lewis and Clark did map the route to the pacific oceans though they were not able to find a direct water route as they had hoped. The Majority of their trip was along the Missouri River and thus it is the closest thing to a "Northwest Passage" that the explorers were able to find.
I would say that the resource you could use to find the meaning of <em>ambiguous </em>is the dictionary.
An almanac is an annual publication about weather, crops, etc.
A thesaurus is a dictionary of synonyms.
An encyclopedia is self-explanatory.