Final answer:
The Kansas-Nebraska Act permitted settlers in the Kansas and Nebraska territories to decide on the issue of slavery, making use of the principle of popular sovereignty. The correct answer is D. allowed people in the Kansas and Nebraska territories to decide whether to allow slavery.
Step-by-step explanation:
The Kansas-Nebraska Act, passed by Congress in 1854, was a significant moment in the nation's history, as it led the country closer to civil war. Contrary to abolishing slavery, the Act allowed the settlers of the Kansas and Nebraska territories to decide for themselves whether to allow slavery, utilizing the principle of popular sovereignty. The decision to repeal the Missouri Compromise line and apply popular sovereignty here increased sectional tensions, undermined party unity, and played a crucial part in the eventual rise of the Republican Party to address the schism over slavery.
Stephen A. Douglas was the architect of the Kansas-Nebraska Act, proposing the bill with the hope of not only promoting party unity and satisfying Southern interests but also with the personal goal of advancing his political career.
Therefore, the correct answer is D. allowed people in the Kansas and Nebraska territories to decide whether to allow slavery, aligning with the principle of popular sovereignty that was one of the Act's central elements. This decision disrupted the established political balance and led to increased sectional conflict, which would soon culminate in violent clashes, notably in what became known as Bleeding Kansas.