Answer:
C. The central idea is that armadillos are the only mammals with protective shells.
Step-by-step explanation:
The National Geographic article "Armadillos" describes how armadillos are one type of mammal that has a feature that makes them unique from others. It details some specific facts about these mammals found mostly in Latin America.
With all specifics about the armadillo, the main or central idea of the article seems to be that they are the only mammals that have protective shells. These sloth-like mammal's shells, bony plates enable them to be secure from any danger.
Thus, the correct answer is option C.