Congress calls for a national constitutional convention
The part of the ratification process in the U.S. Constitution that has never been used is Congress calling for a national constitutional convention. This option involves Congress initiating the process to convene a convention to propose amendments to the Constitution. However, to date, all the amendments that have been added to the Constitution have gone through the first three options mentioned: three-fourths of state conventions ratifying an amendment, three-fourths of state legislatures ratifying an amendment, or Congress voting by a two-thirds majority vote to propose an amendment. Therefore, the correct answer is "Congress calls for a national constitutional convention."
The part of the ratification process in the U.S. Constitution that has never been used is Congress calls for a national constitutional convention.
