ANS:C - ultimate

The factor of safety for brittle materials (having elongation < 5%) is typically based on the ultimate stress. Brittle materials, such as ceramics or certain types of cast iron, do not exhibit significant plastic deformation before fracture. Therefore, their behavior is often characterized by the ultimate stress, which represents the maximum stress the material can withstand before fracturing. The factor of safety is a design criterion used to ensure that structures or components can withstand loads and stresses without failure. For brittle materials, the factor of safety is typically based on the ultimate stress to provide a margin of safety against fracture under applied loads. This ensures that the structure or component will not fail catastrophically under normal operating conditions.