In 1973, President Richard Nixon was linked to a break-in and coverup of the 1972 break at the Democratic National Headquarters at the Watergate complex. Investigations led prosecutors to demand that Nixon hand over all audiotapes recorded in his office, believing that the conversations on the tapes would prove or disprove Nixon's involvement. The President refused, citing "executive privilege" and that releasing the recordings could endanger national security. The prosecution asked the Supreme Court to order Nixon to release all of the tapes, a case that became known as the United States v. Nixon (1974). Based on this information, the case of United States v. Nixon (1974) dealt with what Constitutional principle?

A) Suffrage
B) Federalism
C) Checks and Balances
D) Separation of Powers