With BOLDFACE questions, it's crucial to identify the author's main point - the CONCLUSION - and how all of the other elements of the argument relate to it.

Here, the biggest claim that the author is making is "[this settlement] must have been a major Anasazi center." The "must have been" is our clue that they are drawing a conclusion.

The other sentences in the argument all support the idea that this was a major Anasazi center:

Conclusion: It must have been a major Anasazi center,
because only a major cultural center would have the organizational power to import timber from 50 miles away, and analysis of wood samples shows that some of the timber for the buildings came from Chuska and San Mateo Mountains, 50 miles from Chaco Canyon.

Thus, when we want to describe what roles the boldface portions are playing, we can say:
(1) author's main conclusion / claim
(2) a claim (opinion) that supports the conclusion

(A) The first is premise used to support the argument's main conclusion; the second is the argument's main conclusion.
- the 1st is not a premise
- the 2nd supports the 1st, not the other way around

(B) The first is the argument's main conclusion; the second is a premise used to support that conclusion.
- true
- true

(C) The first is one of the two premises used to support the argument's main conclusion; the second is the other of those two premises.
- the 1st is the conclusion, it's not supporting anything else

(D) The first is premise used to support the argument's main conclusion; the second is a premise used to support another conclusion drawn in the argument.
- the 1st is the conclusion, it's not supporting anything else

(E) The first is inferred from another statement in the argument; the second is inferred from the first.
- the 1st follows from the 2nd (another way of saying that the 2nd supports the 1st), so the first part of this sort of fits
- but the 2nd is not inferred from the 1st. The opposite is true.

The answer is B.