Young’s modulus of elasticity is the ratio of stress to strain, provided the load does not exceed the

Analyze the question: It asks about the condition under which Young's modulus (stress ÷ strain) stays valid. That depends on Hooke's law.

Concept: Young's modulus is defined as

E = stress / strain = (F/A) / (e/L)

This ratio is constant only when the material obeys Hooke's law, which says that extension is directly proportional to the applied force. Hooke's law holds only up to the elastic limit. Up to that point:

• stress is proportional to strain, so the graph is a straight line
• the material returns to its original length when the load is removed

Step by step:

1. Beyond the elastic limit, the material is permanently deformed, so stress is no longer proportional to strain.
2. Past this point, the ratio stress/strain is no longer constant, so Young's modulus is not valid.
3. Check the options: • A. Breaking point: this is where the material fractures, far beyond the valid region. • B. Elastic limit: the maximum load up to which the material recovers fully and Hooke's law holds. ✔ • C. Plastic limit: this is not the standard term for the boundary. • D. Stress limit: not a standard term. • E. Yield point: this lies at or beyond the elastic limit, where the material begins to deform plastically. The strict boundary for Hooke's law is the proportionality limit, which is practically the elastic limit in the NECO syllabus.

Answer: B. Elastic limit

Tip: Remember the order on a stress-strain curve: proportional/elastic limit → yield point → breaking point. "Young's modulus lives only in the straight-line (elastic) region."