Final answer:
The shape of a crossbuck is used exclusively to warn of a railroad crossing and is found in the field of Social Studies at the High School level.
Step-by-step explanation:
The shape of a crossbuck is used exclusively to warn of a railroad crossing in the field of Social Studies. A crossbuck is a type of traffic sign that consists of a white x-shaped or t-shaped panel with a black outline, usually placed at a railroad crossing. It is designed to alert drivers to the presence of a railway and the need to be cautious when crossing.