Final answer:
The number of enslaved people in Mississippi between 1800 and 1860 increased predominantly due to the overseas demand for cotton, which intensified the domestic slave trade after the international trade was banned. (option 1 is the correct answer)
Step-by-step explanation:
What contributed to the increasing number of enslaved people in Mississippi between 1800 and 1860 was primarily the increasing demand for cotton overseas. As the cotton boom took place, the profit-driven cotton industry required an extensive labor force, which was supplied through the slave labor system.
Plantations in the Mississippi River Valley swiftly turned into the centers of cotton production, bolstering the region's economy significantly. The outlawing of the international slave trade in 1808 caused a surge in the domestic slave trade, fueling the demand for enslaved labor to maintain cotton plantations.
Additionally, the introduction of the cotton gin made cotton cultivation more profitable, enhancing the institution of slavery as a backbone for the booming market.