Answer:
The looming national debt is a major problem in the United States that will have untold impact on society.
Explanation:
This is the best thesis statement in the options you have provided as one, it addresses the main topic which is the national debt, while the other 3 talk about the national debt with out actually saying that it is about the national debt. For an example if you had to chose , "the dog is fast or "he is fast" for the thesis statement you should pick "the dog is fast" as it addresses what your talking about. Hope this helps if you have another question comment it.