Final answer:
The Radical Republicans required Confederate states to ratify the Thirteenth Amendment, which abolished slavery, as part of their Reconstruction plan.
Step-by-step explanation:
One element of the Radical Republicans' plan for Reconstruction was requiring Confederate states to approve the Thirteenth Amendment to the Constitution. This was part of various measures to reshape the post-Civil War society and ensure civil rights for formerly enslaved people. The Thirteenth Amendment abolished slavery, and its ratification was a condition for the reintegration of Southern states into the Union. Additionally, the Radical Republicans' efforts included proposals like the Wade-Davis Bill, which mandated that fifty percent of voters in a state must swear loyalty to the Union before the state could be readmitted. However, this particular bill was never enacted into law due to President Lincoln's pocket veto.