Answer:
A). A series of public debates with Stephen Douglas
Explanation:
As per the question, the key cause of Lincoln's nationwide political popularity was 'a sequence of his public debates with the former Democratic senator, Stephen A. Douglas.' The series of seven debates with him regarding the concern over extension of slavery as Douglas had the belief that slavery will automatically come to an end as the product of sovereignty while Lincoln was in favor of a proper law for the abolishment of slavery to outcast it legally. However, the debates could not reach a conclusion yet this point of view made Lincoln extremely popular among the masses.
