The correct answer to this open question is the following.
What finally prompted President Nixon to resign was the revelation of one of the secret tapes that he had ordered to maintain in secret.
Richard Nixon's participation in the Watergate Scandal was definitive. Press reports and investigations revealed that Nixon was part of the scandal. That is why the US Congress decided that Nixon had to be impeached. He would have been the second President of the US to be impeached, but he decided to resign on August 8, 1974.