Final answer:
The English Bill of Rights is the act that reduced the powers of the English Monarch (king/queen) by establishing parliamentary sovereignty and guaranteeing rights and liberties to the people.
Step-by-step explanation:
The act that reduced the powers of the English Monarch (king/queen) is the English Bill of Rights. It was passed in 1689 following the Glorious Revolution, which saw William and Mary become joint monarchs of England. The English Bill of Rights established parliamentary sovereignty, limited the powers of the monarchy, and guaranteed certain rights and liberties to the people.
Learn more about English Bill of Rights and the reduction of English Monarch's powers