Answer: C. Dean was able to testify to dozens of conversations with Nixon about the break-in and cover-up.
Step-by-step explanation:
John Dean was a White House Counsel for President Richard Nixon and in this role he tried to cover-up the breaking into the Democratic Party headquarters at Watergate.
John Dean saw that he might be used as a scapegoat for the coverup and so decided to cooperate with investigators which led to him being fired by Nixon. His testimony was very important because it linked key member of the Nixon administration including Nixon himself to the coverup.