Final answer:
The Kansas-Nebraska Act is the event that best completes the timeline for Bleeding Kansas, as it introduced popular sovereignty to the territories and escalated tensions between pro-slavery and anti-slavery factions.
Step-by-step explanation:
The event that best completes the timeline to show the sequence of events in Bleeding Kansas is b. The Kansas-Nebraska Act. The Kansas-Nebraska Act, proposed by Stephen Douglas, overturned the Missouri Compromise by allowing the possibility of slavery in the Kansas and Nebraska territories through the principle of popular sovereignty. This act led directly to the violent confrontations between pro-slavery and anti-slavery settlers in Kansas, a period known as "Bleeding Kansas," an event that escalated tensions leading up to the Civil War.
Bleeding Kansas served as a precursor to key events that further inflamed sectional strife, such as the Dred Scott Decision and John Brown's raid on Harpers Ferry. These events played significant roles in exacerbating the national divide over slavery, contributing to the rise of the Republican Party and setting the stage for the Civil War.