Final answer:
Stephen Douglas's death had a significant impact on the northern Democrats, as it contributed to a split in the Democratic Party that ensured a Republican victory and the election of Abraham Lincoln in 1860. The correct option is c.
Step-by-step explanation:
The death of Stephen Douglas impacted the northern Democrats significantly. After Douglas's death, coupled with the internal strife over issues such as slavery, the Democratic Party failed to maintain unity and split into factions. This three-way division among Stephen Douglas, John Breckinridge, and John Bell led to a Republican opportunity to win the presidency.
The split in the Democratic Party guaranteed a Republican victory, and as such, Abraham Lincoln won the presidency in the election of 1860. The Democrats' inability to present a unified front due to sectional differences contributed greatly to Lincoln's success. Therefore, Stephen Douglas's death and the subsequent dissolution of unity within the party had significant repercussions for the northern Democrats, leading to their failure in the presidential election.
Hence, Option c is correct.