What shape must the label be for an over-the-road general purpose vehicle powered by LP-gas?

The correct shape for the label on an over-the-road general purpose vehicle powered by LP-gas is a diamond. This specific shape is used for hazardous material identification, as it allows for quick recognition by emergency responders and others who may be involved in safety and regulatory compliance.

The diamond shape is standardized for marking vehicles to indicate that they contain flammable gas, which is crucial for ensuring safety during transport. The bright colors typically used alongside this diamond shape further enhance visibility and aid in conveying important information about the hazardous contents of the vehicle.

In contrast, the other shapes listed do not fit the standardized criteria for labeling vehicles transporting LP-gas. For instance, circles and squares are often used for other types of signage but do not convey the same immediate recognition of hazard that a diamond does in this context. Thus, the diamond shape is essential for clearly communicating that a vehicle is powered by LP-gas, making it the correct choice.