Final answer:
Under President Lincoln's Reconstruction plan, Southerners had to take an oath of loyalty to the US and accept that slavery had ended.
Step-by-step explanation:
Under President Abraham Lincoln's plan for Reconstruction, Southerners had to take an oath of loyalty to the United States and accept that slavery had ended. This was part of Lincoln's intention to reintegrate the Southern states into the Union and outline a policy that would encourage Unionist support within the Confederate states.
The main focus of Lincoln's Ten Percent Plan was quick reunification, requiring a moderate oath of allegiance from Southerners and the acceptance of emancipation, though it left the specifics of ending slavery to be determined by the states themselves. The correct answer to the student's question would be A. slavery.