B. Force per unit area

Here are the answers to the objective questions:

1. B. Force per unit area Stress is defined as the internal force per unit cross-sectional area.
2. B. Change in length divided by original length Strain is a measure of deformation, defined as the change in dimension divided by the original dimension.
3. A. Stress is proportional to strain within elastic limit Hooke's Law states that for elastic materials, stress is directly proportional to strain up to the proportional limit.
4. B. Pascal The SI unit for stress (force per unit area, $N/m^2$) is the Pascal (Pa).
5. A. Stress to strain Young's modulus (or modulus of elasticity) is the ratio of normal stress to normal strain in the elastic region.
6. B. Reversible deformation Elastic deformation is a temporary deformation that is fully recovered when the applied load is removed.
7. C. Deformation becomes permanent Plastic deformation is a permanent change in shape or size that remains even after the load is removed.
8. A. Point where Hooke's law ends The proportional limit is the point on the stress-strain curve up to which stress is directly proportional to strain.
9. B. Dimensionless Strain is a ratio of two lengths (e.g., $\Delta L / L$), so it has no units and is dimensionless.
10. B. Lateral strain to longitudinal strain Poisson's ratio is the negative ratio of transverse (lateral) strain to axial (longitudinal) strain.
11. A. Parallel to the surface Shear stress acts tangentially or parallel to the surface of the material.
12. B. Perpendicular to surface Normal stress acts perpendicular to the surface of the material.
13. A. Force $\times$ distance Bending moment is the product of a force and its perpendicular distance from a reference point.
14. C. One hinge and one roller support A simply supported beam is typically supported by a hinge (pin) at one end and a roller at the other.
15. B. Midspan For a simply supported beam with a concentrated load at its center, the maximum bending moment occurs at the midspan.
16. A. Shear force along beam A Shear Force Diagram (SFD) illustrates how the shear force varies along the length of a beam.
17. C. Bending moment A Bending Moment Diagram (BMD) illustrates how the bending moment varies along the length of a beam.
18. B. N$\cdot$m The unit of bending moment is force times distance, which is Newton-meter (N$\cdot$m).
19. B. Point of contraflexure A point of contraflexure is a point in a beam where the bending moment changes sign, indicating a change in the curvature of the beam.
20. B. Axis with zero stress The neutral axis in a beam subjected to bending is the axis where the longitudinal stress and strain are zero.