What marked the official transition from roman republic to roman empire

President nixon resigned from office in 1974 after he was informed that the senate would impeach him, as allowed in the constitution, for his actions in the watergate scandal. one of the key reasons that nixon’s actions became public was a) the freedom of press guaranteed by the first amendment. b) the right to confront witnesses as guaranteed by the sixth amendment. c) the limitation of judicial powers as defined in article 3 of the constitution. d) the prohibition against illegal search and seizure as guaranteed by the fourth amendment.