<prompt> The proportional limit is defined as:The maximum stress in a structureThe minimum force required to cause a structure to breakThe maximum stress that can be induced without permanent deformationThe maximum elongation under tension that can be measured before failure  <response>: For a material to satisfy Hookes law, the elastic stress must be directly proportional to the elastic strain. The initial region of the stress-strain plot must be a straight line. Because direct proportionality between two quantities is graphically represented by a straight line, the linear portion of the graph in the figure satisfies this law. Because the proportional limit stress corresponding to point P is the greatest elastic stress possible in accordance with this law, it represents the maximum stress above which stress is no longer proportional to strain.Key concept: Proportional limitMagnitude of elastic stress above which plastic deformation occurs. 