Hey Guys,
(A) The difficulty is not in teaching the traditional languages, but in preserving them. The passage never says it's hard to teach a particular language to the children of a tribe. It just says that languages may not be being taught as much, because majority culture renders the language obsolete/isolating.
(B) The passage doesn't say that preserving language is the ONLY way for tribes to hold on to their culture. In fact, culture as a whole isn't discussed much.
(C) There is NO doubt about the value of preserving oral forms of culture, at least as far as this passage is concerned. Some doubts are expressed about preserving those forms through WRITING, because they have been preserved orally for so long.
(D) This is the main idea of the passage. It's simply discussing how certain tribes are trying to preserve language.
(E) This is not the point of the passage. Tribes have been doing that for generations (otherwise, how would the languages still be around?). The INTERESTING part is that many are trying to do it through writing (in other words, they are NOT sidestepping the inherent contradiction mentioned in this answer choice).
I do think the last sentence of the first paragraph is a good marker for the main idea here, though notice that even paragraph 3 discusses the obstacles in the way of "DEVELOPING these teaching methods," rather than the actual obstacles of teaching.
Hope that helps!
-t