Final answer:
The Missouri Compromise allowed Maine to enter the Union as a free state (option C) and settled the issue of slavery's expansion in the territories. (option D)
Step-by-step explanation:
The Missouri Compromise, also known as the Compromise of 1820, brought Missouri into the Union as a slave state and Maine as a free state. It also established a dividing line, known as the 36°30′ line, between slavery and freedom in the Louisiana Purchase lands. Below this line, slavery could expand, while north of it, there would be no slavery. In summary, the Missouri Compromise allowed Maine to enter the Union as a free state and settled the issue of slavery's expansion in the territories.