The Missouri Compromise attempted to resolve regional issues over slavery by adding both a slave state (Missouri) and a free state (Maine) to the country. Therefore, option A is the correct answer. The Missouri Compromise was passed in 1820 and maintained the balance of power between free and slave states in the US Senate by admitting Missouri as a slave state and Maine as a free state. Additionally, it established a boundary at 36°30' north latitude, north of which slavery would be prohibited and south of which it would be allowed. This compromise temporarily eased tensions over slavery, but it ultimately failed to resolve the issue and was repealed by the Kansas-Nebraska Act of 1854.