The correct answer is “Contained an important, unexplained gap of eighteen-and-a-half minutes”.
The Watergate tapes were tapes that recorded conversations between President Nixon and administration officials, family members, White House staff and other. They were produced between 1971 and 1973. First, the Presidente refused to release the tapes, but later when he handled over them, there was an 18 min gap in one of them. This gap was explained by the President’s Secretary Rose Mary Woods, that said that when she was reviewing the tapes she pushed the wrong button and rerecorded this part of the tape. Until today the content of that 18 min gap remains unknown.