The most accurate explanation for why Andrew Johnson was impeached by the House of Representatives in 1868 is:
B. There were many times in which Johnson clashed with Congress, not just one violation of Federal law.
Johnson faced significant opposition from Congress due to his lenient policies towards the South after the Civil War, his opposition to the Reconstruction Acts, and his violation of the Tenure of Office Act when he attempted to remove Edwin Stanton from his position as Secretary of War. This background of consistent conflict with Congress contributed to the decision to impeach him.