The correct answer is D, as it would have stopped slavery in the territory taken in the Mexican-American War.
The Wilmot Proviso was a US law that prohibited slavery in any territory acquired from Mexico during the Mexican War. The conflict over it was one of the main events that led to the American Civil War. Congressman David Wilmot first introduced the Proviso in the House of Representatives of the United States on August 8, 1846, as an amendment to the bill on allocations of $ 2,000,000 for final negotiations to solve the Mexican-American War (this was only three months before the war that lasted two years). It was passed in the House, but it failed in the Senate, where the South had greater representation. It was reintroduced in February 1847 and again approved by the House but failed in the Senate. In 1848, an attempt to make it part of the Treaty of Guadalupe Hidalgo also failed. Political controversies over slavery in the Southwest persisted until the Compromise of 1850.