Nullification was the idea that states could nullify, or void, what? A. The citizenship of any American B. Any federal law it felt was unconstitutional C. Any treaty it felt was unconstitutional D. Any other state's law it felt was unconstitutional

The answer is B. Any federal law it felt was unconstitutional. This is because the states have right to decide if a law is unconstitutional or not. This is because the states intervene if these laws are unfair. Still it is the Supreme Court that decides if it is unconstitutional or not.