Final answer:
Marshall justified the Court's power of judicial review on the grounds that it is within the province of the courts to say what the law is.
Step-by-step explanation:
Marshall justified the claim that the Court has the power of judicial review on the grounds that it is within the province of the courts to say what the law is. This power of judicial review allows the courts to review actions taken by the other branches of government and the states and determine their constitutionality.
In the case of Marbury v. Madison in 1803, Chief Justice John Marshall established the principle of judicial review, asserting the Court's power to review and possibly nullify the actions of Congress and the president.