Final answer:
The correct statement to fill box three in the diagram for the U.S. Constitution amendment process is that the amendment is sent to the states for ratification. This is because the President's signature is not required after Congress proposes an amendment with the necessary majority.
Step-by-step explanation:
The question about the process for amending the U.S. Constitution relates to the necessary steps involved in this critical but challenging procedure. When it comes to the completion of the given diagram, the correct statement to complete box three, coming after the amendment receives a two-thirds vote in both houses of Congress, is that the amendment is sent to the states for ratification. This is because, after Congress proposes an amendment with the required majority, the President's signature is not required; instead, the proposed amendment is sent directly to the states. The process concludes with ratification by three-fourths of the state legislatures or state-ratifying conventions, depending on the method adopted by Congress.
The amendment process is a detailed and structured method, ensuring broad consensus before any changes to the Constitution are made. The requirement for supermajority votes in both houses of Congress followed by ratification from a supermajority of state legislatures creates safeguards against hastily made amendments. This elaborate system has resulted in only twenty-seven amendments being added to the Constitution since its inception, signifying the difficulty and gravity of modifying this foundational document.