ANS:C - Proportional limit.

The proportional limit is a concept in materials science and engineering that refers to the point at which a material ceases to obey Hooke's Law within its elastic deformation range. Hooke's Law states that stress is directly proportional to strain within the elastic deformation range of a material. In other words, as long as the stress applied to a material is below the proportional limit, the material will deform elastically and return to its original shape when the stress is removed. At the proportional limit, the relationship between stress and strain becomes non-linear, indicating that the material has started to deform plastically. Plastic deformation involves permanent changes in the shape or size of the material even after the applied stress is removed. The proportional limit is important because it defines the boundary between elastic and plastic deformation. Below the proportional limit, the material behaves elastically, meaning it can return to its original shape after deformation. However, once the stress exceeds the proportional limit, the material undergoes plastic deformation, and it will not return to its original shape when the stress is removed. It's worth noting that the proportional limit is not always clearly defined and can be difficult to determine experimentally, especially for materials with complex microstructures or behaviors. In practice, engineers often use the yield strength, which is a more practical and reliable measure of a material's resistance to plastic deformation, as a design parameter.