Abraham Lincoln became a nationally known political figure based on a series of public debates with Stephen Douglas.
his victory over Douglas for a Senate seat.
his retirement from the House of Representatives.
his support of popular sovereignty.

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.

a countries leader should be motivated by the common good of the country.

the monroe doctrine is the best known u.s. policy toward the western hemisphere. buried in a routine annual message delivered to congress by president james monroe in december 1823, the doctrine warns european nations that the united states would not tolerate further colonization or puppet monarchs.