The prominent figure that opposed the Mexican-American War was (C) Henry Clay.
Henry Clay was a U.S. congressman and a presidential candidate that was also called the "Great Pacificator". He was against the annexatiation of Texas because it meant to risk the integrity of the Union; however, after some time, he decided that he was willing to support the annexation if this was achieved without a war. Therefore, in contrast to James Polk, who had an expansionist agenda, Henry Clay opposed the Mexican-American War. The declaration of war was finally approved by the U.S. Congress on May 13, 1846.
