(C) says, "Aboriginal communities should be granted full protection of all of their customs."
The big problem that the author highlighted with respect to extending protection to aboriginal customs dealt with the documentation that the aboriginal people needed in order to prove that their various customs were actually traditional (lines 36 to 39). Although (C) would solve the problem of documentation since aboriginal people wouldn't have to prove that their customs were old, it is too strong of an answer. The reason it's too strong is that it's possible that the author is actually okay with limiting the constitutional protections to only old customs, as we have no evidence that the author is mad about the fact that the protection is only extended to indigenous customs. Thus, we do not know whether the author would agree with this strong of a proposal, so we can get rid of (C).
(B) says, "Oral tradition should sometimes be considered legal documentation of certain indigenous customs."
From the passage, we know that the author recognized two main problems with how aboriginal rights are currently handled. The first is that the oral tradition used by various aboriginal groups made it hard to prove that certain types of customs were actually indigenous (lines 36 to 29), and the second is that courts haven't yet granted full land rights to certain aboriginal groups (lines 51 to 56). From this, we can infer that the author of the passage would likely agree with a proposal that'd help solve either of these problems. (B) talks about the big problem of using oral tradition to prove that customs were indigenous. Treating oral tradition as a sort of legal documentation of various indigenous customs would certainly make it less difficult for various aboriginal groups to prove that they've done certain types of customs for long periods of time, and would thereby reduce the problem that the author described in the second paragraph of the passage. Thus, (B) is the correct answer choice.
Does this make sense? Let us know if you have any more questions!