Which sign indicates the route of the U.S. highway?

Answer: A — black square with a white shield

The correct sign indicating the route of a U.S. highway is characterized by a black square with a white shield. This design is specifically used to denote the U.S. highway system. The shield shape is a recognized symbol for these routes, helping drivers easily identify the highway as part of a national network of roads. The black square background enhances the visibility of the white shield, making the sign stand out against other road signs. In contrast, the other signs mentioned serve different purposes. The blue rectangle with numbers typically indicates interstate highways or routes related to services, while the green circle might be used for local or specific route signs but does not denote U.S. highways. The white x with the words "U.S. Route" is not a standard sign for indicating U.S. highways, making the black square with the white shield the clear and appropriate choice. Understanding these distinctions is crucial for safe navigation on the roads.