The correct answer is the Compromise of 1850.
The Compromise of 1850 was passed shortly after the end of the Mexican-American War. After this war, the United States gained significant territory from Mexico including modern day California, New Mexico, Arizona, etc. However, the question became whether or not slavery was going to exist in these new territories.
The 36'30 line established in the Missouri Compromise was no longer of help, as California would be a half free half slave state if this line stood. So, instead California entered as a free state and the territories of New Mexico and Utah got to vote on whether or not slavery would exist when they were admitted into the Union. This idea of popular sovereignty officially voids the Missouri Compromise.
