Final answer:
Option (d), The most crucial event leading to the Civil War was Abraham Lincoln's election in 1860, as it caused a drastic sectional split and directly precipitated the secession of Southern states.
Step-by-step explanation:
Among the listed events, d) 1860 - Abraham Lincoln's Election was the most crucial in leading the nation to war. Lincoln's victory signified a deep divide between the Northern and Southern states over the issue of slavery expansion into new territories. Despite Lincoln's assurance to not interfere with slavery where it already existed, his position against the expansion of slavery was clear, and his election was the final straw for Southern states. The secession of South Carolina, followed by other Southern states, positioned the nation on the brink of civil war, ultimately leading to the conflict commencing in 1861.
Previous events such as the Kansas-Nebraska Act and the Dred Scott decision increased sectional tensions but did not immediately precipitate war. Lincoln's election, coming after a decade of rising hostilities and failing compromises, acted as the catalyst for the secessionist movement that triggered the American Civil War.