For the United States federal judicial system, the number of justices and the number of judges are controlled by Congress. Since Congress is the legislative branch of the United States government, the best answer to this question is Option B.
While we tend to think that the number of Supreme Court justices is set by the Constitution, it actually is not. That is why, for example, President Franklin D. Roosevelt was able to propose his “court packing plan” to increase the number of justices on the Court. The Constitution says nothing about how many justices the Court includes. Furthermore, the Constitution clearly says that Congress gets to control how many federal judges there are in courts below the Supreme Court. As Article III of the Constitution says, there will be a Supreme Court and “such inferior courts as the Congress may from time to time ordain and establish.” Clearly the Constitution leaves it up to Congress to decide how many of these courts there will be and how many judges will be on each court.
Thus, Option B is the correct answer.