The correct answer to this question is the following.
Although there are no options attached, we can say the following.
At the time of Missouri's admittance to the Union, there was another admittance to the union. It was the state of Maine.
So we can say that the admission of Maine into the Union at the same time as Missouri represented a significant provision of the Missouri Compromise to maintain the balance of slave states and non-slaves states. In other words, the slave-holding states and the free states of the Union wanted to maintain a balance of control and power in the representation in the Congress of the United States. These states knew that any imbalance regarding this issue could affect their political and economic interests.
So Main entered as a free slave state, meanwhile, Missouri entered as a slave state.