Choice C is the best answer. "Protect" means "preserve" or "keep safe from." By ensuring that the wilderness areas along the Appalachian Trail can’t be sold or developed, the National Trails System Act protects them.

Choice A is incorrect. "Borrow" means "to take something with intent to return it." The text doesn’t say anything about taking and returning the wilderness that surrounds the Appalachian Trail. Choice B is incorrect. "Postpone" means "to put off until later." Nothing in the passage suggests that Congress wants to "postpone" the wilderness areas (and that doesn’t make sense anyway—they can postpone doing something to the wilderness areas, but they can’t postpone the areas themselves). Choice D is incorrect. "Decorate" means "to adorn" or "add extra items or pictures to make more attractive." No details in the text suggest that Congress wishes to make the trail fancier or more attractive.