Which answer best describes how southern states were admitted back into the united states?

each state had to agree to follow the civil rights act of 1866.
each state had to ratify the thirteenth amendment of the constitution.
each state had to ratify the fourteenth amendment of the constitution.
each state had to sign a document stating they would never secede again.

Each state had to ratify the Fourteenth Amendment of the Constitution."By the summer of 1868 the legislatures of 7 (of 11) Southern states had approved the Fourteenth Amendment. The remaining four states-Georgia, Mississippi, Texas, and Virginia-complied with the requirements for statehood by 1870, at which time the Union was restored and Congressional representatives from the South were again welcomed in Washington."