Which structural unit is specifically made up entirely of triangles in a flat plane?

The correct answer revolves around the unique characteristics of lightweight wooden trusses, which are structural units composed entirely of triangular shapes arranged in a flat plane. This design is key because triangles are inherently strong shapes that distribute loads evenly across their structure, making them highly efficient in terms of both material usage and structural integrity.

Lightweight wooden trusses utilize this triangular configuration to create a rigid framework that can span long distances without the need for intermediate columns. This is particularly beneficial in both residential and commercial construction, allowing for open floor plans and maximizing usable space.

In contrast to lightweight wooden trusses, other options like lightweight steel trusses also employ triangular shapes, but the question specifically asks for a structural unit composed entirely of triangles in a flat plane, which emphasizes the wooden variant in this context. Composite beams and metal girders, while crucial in construction, do not rely on a triangular shape in the same way and often involve more complex geometries or a combination of materials that do not maintain the same uniform triangular pattern throughout.