Answer:
The free state that was admitted to the Union with the Missouri Compromise was Maine.
Explanation:
The Missouri Compromise was an agreement taken in 1820 between the representatives of the slave and abolitionist states in the United States Congress in relation to the regulation of slavery in the Western territories, which in the future would become states, to maintain the majority, or equality at least, of the number of states opposed to slavery, existing from the creation of the United States until then.
The compromise arose from the need to maintain the balance that existed at the time between the 11 non-slave states and the other 11 slave states, which would be disbalanced with the admission of the new state of Missouri, slave, in 1819. This would unbalance the composition of the Senate in favor of the slave states. In the House of Representatives there was no such balance because its representatives were elected proportionally to the population, which was larger in the northern states.
The negotiated solution that was reached, on March 2, 1820, thanks to the proposal of Senator Henry Clay, consisted in admitting the state of Missouri as a slave state, while creating the state of Maine, (which until then depended on the state of Massachusetts), as a non-slave state.
