The ratio of change in length to original length under stress is Strain.

• A. Strain
• B. Stress
• C. Elastic modulus
• D. Poisson's ratio

Answer: (A)

Strain measures how much a material deforms relative to its original size. It is defined as the change in length divided by the original length: ε = ΔL / L0. This makes it a dimensionless quantity that can be expressed as a decimal or a percent. The statement matches this idea since it’s exactly the ratio of the length change to the starting length when the material is under load.

Stress, by contrast, is about the external force applied per unit area (not about deformation itself). The elastic modulus (like Young’s modulus) connects stress and strain in the elastic region with σ = Eε. Poisson’s ratio relates the lateral (transverse) strain to the axial (longitudinal) strain, ν = -ε_trans / ε_axial.

Example: if a rod of 2 meters length stretches by 0.02 meters, ε = ΔL/L0 = 0.02/2 = 0.01, or 1%.