Why was missouri joining the union controversial? missouri wanted to be a free state, so the south did not want to admit it to the union. missouri would upset the balance between the equal number of slave states and free states. missouri would be too much of a drain on the national economy, so both the north and south opposed statehood. missouri would cause there to be twice as many slave states as free states, so free states opposed statehood.

The correct option is this: Missouri would upset the balance between the equal number of slave trades and free states.
In 1820, when Missouri was about to be admitted into the union, the issue of slavery came up again, because there had been tension between the north and the south concerning the issue of slavery. At that time the number of states who participate in slavery and those who did not were equal. But when Missouri applied to the union as a slave state, there was controversy because its admission will upset the balance of free and slave states.