Answer: D. stop slavery in the territory taken in the Mexican-American War
The Wilmot Proviso was created to stop slavery in all its forms, in a number of key areas. These areas covered any and every territory that was taken from Mexico during the Mexican-American War. The proviso, in hindsight, would have gone a long way in helping to prevent the Civil War.
William Proviso did not have any information on returning New Mexico to Mexico, nor did it aim to make California into a slave state. It had nothing to do with stopping the distribution of war pay to soldiers from the Mexican-American War. Instead, it was focused on stopping slavery in Mexico.