C) discovery of audio tapes of conversations connecting him in attempts to cover up details of the Watergate break-in.
Explanation:
The Watergate scandal took place from 1972 to 1974. It describes a break-in by five men at the Democratic National Committee headquarters in Washington, D.C. in 1972. More importantly, the name is applied to Nixon's attempts to cover up his involvement. This was one of the greatest scandals in American history, and it caused Nixon's downfall.
Sources such as diaries though not very reliable can be used if there are a large amount of them showing the same date and event. Historical books written about the event especially those closer to the time would be able to help determine that the date is accurate as well.
