Question

Why did the North fear would happen if Missouri became a slave state?

slavery would not be able to expand to other parts of the country
no other states would be admitted to the Union
slavery would be forced upon the all the Northern states
the rest of the Louisiana Territory would become slaveholding territory

Answer 1

The rest of the Louisiana Territory would become slaveholding territory.

They feared admitting Missouri as a slave state would set a precedent and allow slavery to spread westward, upsetting the balance of power in the Senate.