Final answer:
The Mexican Cession, resulting from the Treaty of Guadalupe Hidalgo in 1848, sparked debates over the expansion of slavery into new territories, leading to escalating tensions between the North and South. Hence, the correct answer is option (C).
Step-by-step explanation:
The territorial addition that sparked questions of how slavery would spread in the United States during the early 19th Century was C) The Mexican Cession. This was due to the Treaty of Guadalupe Hidalgo in 1848 which added a significant amount of territory to the United States.
Southern expansionists were enthusiastic supporters of territorial gains like the Louisiana Purchase and actively pursued adding more slaveholding territories through events such as the annexation of Texas and the Mexican-American War. The Mexican Cession raised intense debates between the North and South over whether new states formed from these territories would institute slavery or remain free.
The tensions were not fueled by humanitarian concerns as much as they were by the economic and political implications of permitting slavery to expand into newly acquired western territories. This culminated in various legislative attempts to address the issue, including The Compromise of 1850, but ultimately led to increasing sectional tensions that played a role in leading the nation towards the Civil War.