Final answer:
The final step for ratifying an amendment to the Constitution after proposal by Congress is for three-fourths of the states to ratify the amendment within a set deadline.
Step-by-step explanation:
The last step in ratifying an amendment to the Constitution of the United States, after being proposed by a two-thirds vote in both the House of Representatives and the Senate, is c. the article must be ratified by three-fourths of states within a proscribed deadline. This ratification process does not include a case before the Supreme Court, the signature of the President, or a two-thirds state ratification threshold. The requirement for a three-fourths majority ensures broad consensus among the states, and it represents a high bar for change, reflecting the significance of amending the foundational document of the nation's governance.