Final answer:
The Missouri Compromise balanced the number of free and slave states by admitting Missouri as a slave state and Maine as a free state, and banning slavery north of the 36°30′ latitude in the Louisiana Territory. Answer B is correct, as it established a requirement for maintaining the balance in the Senate.
Step-by-step explanation:
The Missouri Compromise maintained the balance between free states and slave states by providing a legislative solution that admitted Missouri as a slave state and Maine as a free state.
Simultaneously, it drew a geographical line across the Louisiana Territory at the latitude 36°30′, north of which slavery was prohibited, except for Missouri. This maintained an equal number of slave and free states in the Senate, keeping a delicate political balance.
The correct answer to the question of how the Missouri Compromise maintained the balance between free states and slave states is B. It required a new slave state to be added for every new free state.
This element of the compromise was crucial to preserving the equilibrium in the Senate, where each state is represented by two senators, regardless of population size.
Furthermore, the Thomas Proviso, which was included in the compromise, banned slavery in future states to be created north of the 36°30′ line within the Louisiana Purchase, except for Missouri. This limitation also helped to cap the spread of slavery in the new territories of the United States.