Strain from stress and Young's modulus Steps:

• Tensile force F is known and acts within the limit of proportionality, so Hooke's law applies.
• Stress σ = F / A, where A is the cross-sectional area.
• Young's modulus E = σ / ε, so strain ε = σ / E = (F / A) / E = F / (E A).
• Measure A and use known E to compute ε.

Why B is correct:

• Strain ε = F / (E A); with F known, measuring A and using material property E enables direct calculation via the definition of Young's modulus.

Why the others are wrong:

• A: Provides A for stress σ = F / A but lacks E to find ε = σ / E.
• C: Provides E but lacks A to find σ = F / A, so ε cannot be computed.
• D: Provides A for σ but lacks E to find ε = σ / E; stretched length adds no value for strain.

Final answer: B