America is a nation, loosely speaking - although there could be a dispute as to whether it refers to the US or the whole of N and S America. I don't know how inhabitants of the US refer to themselves; I suppose as Americans. They will tell us.
You could say "The United States spends more on fast food than any other country."
B in the OP doesn't work for me either.
"The United States spends more on fast food than any other country," isn't awkward or wrong in itself, but without any context it could be taken to mean the US Federal government.
Yet another possibility: "People in the United States spend more on fast food than any other nation."
Sure, comparing like with like is always good, I didn't mean this was the only way to express the idea. A people can constitute a 'nation' in informal speech* and ordinary, relatively casual English doesn't always make exact grammatical correspondences. One could say "People in the United States spend more on fast food than people in any other nation," but someone could sarcastically counter that with "Who else in other nations is going to spend the money? Dogs?"
In my post #9, I used 'people' to get around the possible interpretation that some part of the government of the US was buying the fast food: "The United States imposed sanctions on Russia" refers to the government, not the inhabitants.
*For instance, Red Sox Nation = the fans of the baseball team called the Red Sox. Traditionally, it refers to the residents of most of the six New England states (except for the southwestern part of Connecticut) plus some parts of northern New York State: that is, the northeast part of the US that is not under the influence of New York City.