Final answer:
Southern States objected most to the Radical Republicans' Reconstruction requirement to ratify the 14th Amendment for readmission to the Union, as it promoted civil rights for formerly enslaved individuals.
Step-by-step explanation:
The provision of the Radical Republicans' plan for Reconstruction that Southern States found most objectionable was that a former Confederate State could not be readmitted to the Union unless that State ratified the 14th amendment.
The Radical Republicans were a group within the Republican Party that sought federal government intervention in the process of Reconstruction. They played a crucial role in influencing Reconstruction policy after the Civil War, with a strong commitment to ensure that the rights of freed slaves were protected and that genuine Unionists had control. One of the pivotal conditions they established for the readmission of former Confederate states to the Union was the ratification of the 14th Amendment, as part of their broader efforts to reshape Southern society and extend citizenship and equality to the recently freed people. This amendment addressed citizenship rights and equal protection, which was central to the Radicals' goal of civil rights for formerly slaves and guaranteed no state would unlawfully infringe upon these rights. Southern states, keen to restore their place in the Union but resistant to such fundamental changes, found this particular provision one of the most difficult to accept.