How many justices must agree to hear a case in the Supreme Court?

The correct answer is that four justices must agree to hear a case in the Supreme Court. This principle is known as the "rule of four." It allows a minority of the justices to grant a writ of certiorari, which is an order by the Supreme Court to review a lower court's decision. This rule is designed to ensure that cases of significance, which may not have the backing of a majority of justices, can still be considered by the Court.

This mechanism supports the Court's role in addressing important legal issues and allows for differing judicial perspectives to be represented, especially in cases where fundamental rights or interpretations of law are at stake. The majority of justices do not need to agree to take on a case, allowing for flexibility in the types of cases that the Court can address and ensuring that significant legal questions can be explored even if they do not attract broad consensus among the justices.