Which of the following was established by a constitutional amendment?

The statement that the President may serve no more than two elected terms is correct and stems from the 22nd Amendment to the United States Constitution, ratified in 1951. This amendment was established in response to Franklin D. Roosevelt's four terms in office, which raised concerns about the potential for excessive executive power and the lack of checks on presidential authority. The 22nd Amendment limits individuals to being elected as President for a maximum of two terms, thereby promoting the idea of regular leadership change and preventing any one person from holding the presidency for an extended period.

The other options relate to functions and operations of government but do not originate from a constitutional amendment. For instance, treaties are ratified by the Senate regardless of how they are negotiated, which is a long-established practice rather than an amendment. The matter of congressional bills being signed by the President is also governed by long-standing legislative procedures rather than an amendment. Lastly, the term limits for senators are not equally defined within the Constitution, as there's no constitutional amendment that limits their terms.