Final answer:
A two-thirds majority of Congress is the best example of a check on the ability to propose a constitutional amendment, reflecting the significant consensus needed for such a critical action.
Step-by-step explanation:
The best example of a check on who can propose an amendment to the Constitution is a two-thirds majority of Congress.
This process is part of the Amendment Process where both the House of Representatives and the Senate must approve a joint resolution by a two-thirds supermajority vote, as outlined in Article V of the Constitution.
The president does not have a role in proposing amendments, and the Supreme Court is not involved in the proposal process but may be involved in interpreting the Constitution or the validity of the ratification process.
Achieving a two-thirds supermajority is a restrictive measure that ensures significant consensus is needed to propose changes to the foundational legal document of the United States.