What best defines nullification? A. Congress passed the protective Tariff of 1816. B. Senator Henry Clay proposed the "American System". C. Southern senators Calhoun and Hayne argued that Congress could not pass laws that favored one region of the country over another.

Nullification, in United States constitutional history, is a legal theory that a state has the right to nullify, or invalidate, any federal law which that state has deemed unconstitutional. So the best answer among the following choices is letter C. Southern senator Calhoun and Hayne argued that Congress could not pass laws that favored one region of the country over another.