A

C. The extent to which the separation of powers was allowed.

Explanation:

The United States v. Nixon (1974) involved the President Richard Nixon who was accused of being involved in the Watergate Scandal (1972). The Watergate Scandal was a break-in to the headquarters of the Democratic National Committee that revealed that the Nixon administration had abused its power multiple times.

The President was asked to provide the documents and the unedited tape recordings related to the incident but refused to do so, claiming that he had a right to withhold data from other government branches. The Supreme Court determined that the executive privilege was limited and forced Richard Nixon to provide evidence. As a result, the President resigned.

Newspaper page about President Richard Nixon’s resignation.
Newspaper page about President Richard Nixon’s resignation (Source: www.openculture.com)