Young's modulus definition

Steps:

• Recall Young's modulus E equals stress divided by strain.
• Stress is force F divided by cross-sectional area A, so stress = F/A.
• Strain is extension x divided by original length L, so strain = x/L.
• Substitute to get E = (F/A) / (x/L) = (F/A) × (L/x), or directly (F/A)/strain.

Why B is correct:

• B expresses E as stress (F/A) divided by strain, matching the formula E = stress/strain from Hooke's law.

Why the others are wrong:

• A: Stress × strain gives energy density, not Young's modulus.
• C: F x / A L equals stress × strain, which is incorrect for E.
• D: Uses general terms but doesn't specify the variables F, A, x, L as required.

Final answer: B