The items that were key to the Missouri Compromise were laws that admitted that Maine was a free state and Missouri a slave state. Hence, the correct answer is B.
This refers to the Law of 1820 which was passed to declare that or admit that Missouri was a state that practiced slavery and Maine - a free state.
It took 30 years before the law would be repealed by the Kansas-Nebraska Act of 1854.
See the link below for more about the Missouri compromise:
https://brainly.com/question/8140181