What reference tool is used to determine fiber orientation for a ply of fabric?

The correct answer is the warp clock, also known as a compass. This tool is specifically designed to determine the fiber orientation of fabric in composite materials. Understanding fiber orientation is crucial in the fabrication of composite structures, as it significantly influences the mechanical properties and performance of the finished product. The warp clock provides a visual representation of the ply's orientation in relation to the fabric's geometry, making it easier for technicians and engineers to lay the fibers at the desired angles for optimal strength and flexibility.

In contrast, the other tools listed serve different purposes. A protractor measures angles but does not offer specific guidance for fiber orientation in fabrics. A fiber orientation manual might provide guidelines and references but does not physically indicate the orientation itself. A caliper is used for measuring the dimensions of objects, such as thickness or gaps, but is not related to fiber orientation. Therefore, the warp clock is the most appropriate and specialized tool for determining the fiber orientation in a ply of fabric.