Final answer:
The ultimate authority in the Islamic Republic of Iran's constitution is the Supreme Leader, a high-ranking Shia cleric with extensive political and religious power.
Step-by-step explanation:
The ultimate authority, as written into the Islamic Republic of Iran constitution, is the Supreme Leader, who is a high-ranking Shia cleric with both political and religious authority over the nation. Following the Iranian Revolution of 1979 and the establishment of the Islamic Republic, Islam was declared the official religion of Iran and all laws were required to conform to Islamic law. The constitution and political system of Iran reflect a unique blend of religious rule and elements of democracy, as seen in the role of the Supreme Leader, who has significant influence over all branches of government and is considered to have the final word on matters of state and religious doctrine. This role was established after the overthrow of the Shah, subsequent to the return of Ayatollah Khomeini, and has remained a central figure in Iran's governance ever since.