Answer: The right answer is the B) "He has been taught by our traditional people that there are thirteen full moons in a year."
Explanation: Just to elaborate a little on the answer, it can be added that ethos is one of the three persuasion devices—the other two being pathos and logos—and it is aimed at convincing the reader of the credibility of the author (or of his/her source, or the person he/she is talking about) and, eventually, persuade him or her of that author's argument. In this paragraph, the author is describing in which ways one of the members of his tribe is culturally "different." By indicating that "their traditional people" taught him, the author is stressing that highly respected and knowledgeable people taught him what he knows (among other things that there are thirteen full moons in a year according to the native tribal calendar, and there are really thirteen planets in our solar system), and that makes him arguably different, but not disadvantaged.