Final answer:
Justices in the Supreme Court have the power to determine the constitutionality of actions through judicial review, ensuring laws and government actions adhere to the Constitution.
Step-by-step explanation:
The institution in the Supreme Court that has the ability to decide whether an action is unconstitutional or not is the justices. Justices are responsible for exercising judicial review, which is a critical function in our legal system as it allows the judiciary to strike down laws and other government actions that it finds do not align with the Constitution.
Judicial review ensures that the other branches of government do not exceed their constitutional authorities and that individual rights are protected. The Supreme Court's power to declare acts unconstitutional enables them to serve as a check on the legislative and executive branches, maintaining the balance of power within the government.