Final answer:
The writing of the Constitution was the most important event/action in transitioning the colonies to a democratic republic.
Step-by-step explanation:
The event/action that I believe was the most important in transitioning the colonies from a monarchy to a democratic republic is d) The writing of the Constitution.
While all the events/actions mentioned were significant in their own right, the writing of the Constitution laid the foundation for the governance and structure of the newly formed United States. The Constitution established the separation of powers, the protection of individual rights, and the framework for a democratic system of government.