The answer is: A. Because his culture teaches that there are 13 full moons in a year
This excerpt from “An Indian Father's Plea” By Robert Lake show that he believes that are 13 months because there are also 13 full moons.
“He is not culturally "disadvantaged," but he is culturally "different." If you ask him how many months there are in a year, he will probably tell you 13. He will respond this way not because he doesn't know how to count properly, but because he has been taught by our traditional people that there are 13 full moons in a year according to the native tribal calender and that here are really 13 planets in our solar system and 13 tail feathers on a perfectly balanced eagle, the most powerful kind of bird to use in ceremonial healing.”