Which event completes this timeline?

The correct event that completes the timeline is that the Constitution becomes effective once enough states ratify it. So, D. is the best choice.
What does constitutional ratification entail?
A convention that has been called for that purpose or one that is approved by two-thirds of the States, or by a vote of both Houses of Congress, may propose an amendment. The amendment must then be approved by three-fourths of the State legislatures or three-fourths of the ratification conventions called in each State. The populace needed to be made aware of the proposed new government's provisions, so they could participate in the ratifying conventions. The drafting of the Constitution began on May 25, 1787, when the first quorum of the Constitutional Convention convened at the Pennsylvania State House in Philadelphia to discuss the Articles of Confederation, and it was completed on September 17, 1787. The Constitution's ratification process then started on that day and ended on May 29, 1790, when Rhode Island became the last state to do so.
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the event completes this timeline is the Constitutional Convention held in Philadelphia.
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