The correct answer is D) the Missouri Compromise.
The event that occurred first was the Missouri Compromise.
In 1820, many conflicts divided the states of the Union. The Northern states were against slavery and supported abolitionism. The Southern states relied on slavery to produce cash crops in the large plantations that had to be exported to Europe. So they supported the use of slaves. So the US Congress had to make a decision that wouldn't create more problems. That's how Congres came up with the Missouri Compromise. Missouri was accepted as a slave state but at the same time, Maine was accepted as a free state.