What was the final state to ratify the constitution in may of 1790?

The answer is: Rhode Island

Explanation:

This state opposed federal control of currency and was critical of compromise on the issue of slavery, resisted ratifying the Constitution until the U.S. government threatened to sever commercial relations with the state. On May 29, 1790, Rhode Island voted by two votes to ratify the document, and the last of the original 13 colonies joined the United States.