Which of the following was true of the judiciary act of 1801? a. it packed the supreme court with democratic republicans. b. it reduced the power of the federal courts. c. it was opposed by the federalists. d. it expanded the federal court system.

Answer D is correct. : it expanded the federal court system.
 
Until this Act, state courts had a bigger share of the federal juristiction. The Judiciary Act strengthened the federal court.
The Answer B is wrong, since it is, in a way, the opposite of D. 
To understand the reason for rejecting the other answers, some context is needed:

The Judiciary Act of 1801 is also known as the Midnight Judges Act, because it was passed during the last days of the presidency of John Adams, a Federalist, when it was already known that the next president would be Thomas Jefferson, a Democratic Republican. Adams was accused of trying to fill the judgeships created with this act with fellow Federalists. Thus the asnwer A is wrong: the opposite was true, as this act increased judgeships for Federalists, not Republicans. Therefore it was also not opposed by the Federalists (but the Democratic Republicans (so answer C is wrong).