D) an informal amendment.
When President William Henry Harrison died, Vice President John Tyler took the oath of office to become president. This is an example of an informal amendment to the Constitution, as it was not a formal change to the document but rather a change in the way that it was interpreted and applied. Informal amendments can occur when the Constitution is interpreted in a new way or when new practices or customs develop that are not explicitly mentioned in the Constitution but are consistent with its principles. In this case, the informal amendment was the adoption of the practice of presidential succession, which allows the vice president to assume the office of the president in the event of the death or incapacitation of the incumbent. This practice is not explicitly mentioned in the Constitution, but it has become an accepted and important part of the presidential succession process.