Explanation:

M/EI is a term used in the theory of flexure, which deals with the bending of beams. In simple terms, it represents the curvature of a beam under a bending load.

- M: Moment of the bending load applied to the beam
- E: Modulus of elasticity or Young's modulus of the material of the beam
- I: Moment of inertia or second moment of area of the beam's cross-section

Here's why the correct answer is option 'C':

- Stress: Stress is the internal force per unit area that resists deformation of a material. While bending, the stress varies along the cross-section of the beam and depends on the distance from the neutral axis. Therefore, M/EI is not a measure of stress.
- Rigidity: Rigidity is the ability of a material to resist deformation under an applied load. It is represented by the modulus of elasticity. M/EI is not a direct measure of rigidity, although it indirectly affects the rigidity of a beam by determining its deflection under a load.
- Curvature: Curvature is the amount by which a beam deviates from a straight line when subjected to a bending load. M/EI is directly proportional to the curvature of a beam, with a higher value indicating a greater curvature.
- Shear force: Shear force is the internal force that acts parallel to the cross-section of a beam and causes it to shear or slide apart. It is not related to M/EI, which only deals with the bending behavior of a beam.

In conclusion, M/EI is a measure of the curvature of a beam under a bending load.