(B) says, "It is an intermediate conclusion made plausible by the description of the terrain along which the migration supposedly took place."
The problem with (B) is that the description of the terrain in the stimulus supports the idea that the route was attractive, but it does not support the claim that the migration occurred. Thus, we can get rid of (B).
(C) says, "It is offered as evidence in support of the claim that trade routes between China and the West could easily have been established much earlier than is currently believed."
(C) looks great. The claim in question is provided by the passage's author as evidence, and the claim it is used to support is correctly described in (C). This makes (C) the correct answer choice.
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