C) His opposition to Radical Reconstruction policies for former Confederate states was the main cause of the impeachment of President Andrew Johnson in 1868. Specifically, Johnson was accused of violating the Tenure of Office Act, which was passed in 1867 by the Radical Republicans in Congress to limit the president's power to remove certain officeholders without the Senate's approval. Johnson had dismissed the Secretary of War, who was a supporter of Radical Reconstruction policies, without seeking the Senate's approval. This led to his impeachment by the House of Representatives and subsequent trial in the Senate, where he was acquitted by one vote.