When the assigned altitude in a previously issued clearance is amended, what must the controller restate?

When an assigned altitude in a previously issued clearance is amended, the controller must restate the applicable altitude restrictions. This is crucial because the pilot needs to be clearly informed about the new altitude that has been assigned, ensuring safe separation from other aircraft and compliance with air traffic control directives. Clear communication regarding altitude changes helps prevent misunderstandings that could lead to altitude deviations or potential safety issues during flight. It's essential for maintaining the integrity of the flight's vertical navigation and the overall safety of operations in controlled airspace.

The other choices involve elements of flight that may not necessarily change with an altitude amendment. New speed restrictions, departure airport information, and route changes might be relevant to the flight, but they are not directly tied to the specific requirement of restating altitude instructions when an altitude amendment occurs.