Question

the constitutional question at the heart of united states v. nixon is best described as
whether the supreme court was part of the executive branch.
the right of a president to resist impeachment.
the extent to which \separation of powers\ was constitutional.
whether laws had been broken during the watergate incident.

Explanation:

United States v. Nixon centered on the scope of executive privilege and how it interacts with the separation of powers. The core constitutional issue was defining the limits of presidential authority relative to the judicial branch, which ties to the extent of the separation of powers principle in practice. The other options are incorrect: the Supreme Court is part of the judicial branch (not executive), the case did not focus on resisting impeachment, and it was not about whether Watergate laws were broken but about executive privilege and separation of powers.

Answer:

the extent to which "separation of powers" was constitutional.