The correct answer is:
Abuse of power, Obstruction of justice and Contempt of Congress.
On July 1974, the House Judiciary Committee issued to the House of Representatives three articles of impeachment against Nixon, for obstruction of justice, abuse of power, and contempt of Congress.
Previously, a subpoena from the Judiciary Committee demanded edited transcripts of Watergate-related conversations from the Nixon White House tapes to be made public, but Nixon refused to present the full tapes and additional conversations, so the Supreme Court ordered him to comply.