- The slope of the linear portion of a stress-strain curve represents the material's stiffness or rigidity.
- This linear region corresponds to the elastic behavior of the material, where the deformation is reversible upon removal of the applied stress. The property indicated by this slope is known as the Young's modulus, also referred to as the Elastic modulus.

- Young's modulus is a fundamental mechanical property that measures the ability of a material to resist elastic deformation under load.
- Mathematically, it is defined as the ratio of stress (force per unit area) to strain (proportional deformation) within the elastic limit:
Young’s Modulus (E) = Stress / Strain

- Poisson's ratio (Option 1) relates lateral strain to axial strain but does not describe the slope of the stress-strain curve.
- Yield strength (Option 3) marks the transition from elastic to plastic deformation and is represented by the end of the linear region.
- Toughness (Option 4) describes the total energy absorbed before fracture, represented by the area under the entire stress-strain curve, not just the slope.

In summary, the linear portion slope effectively demonstrates how much a material will deform elastically under applied stress, which is quantified by the Young's modulus (Elastic modulus).

Reference: Materials Science and Engineering: An Introduction, 10th Edition, by William D. Callister Jr., Chapter 5, Page 170