\n\nThe area under the stress-strain curve from O to A (the linear elastic portion) physically represents the
Awork done by the external force per unit area
Belastic potential energy stored per unit volume
Ctotal energy lost to heat per unit length
Dtoughness of the material
Answer & Solution
Correct answer: B. elastic potential energy stored per unit volume
For a linear-elastic deformation, work done on a unit volume equals the area under the $\sigma$–$\varepsilon$ curve, $u = \tfrac{1}{2}\sigma\varepsilon$. All of this is stored as recoverable elastic potential energy (per unit volume). 'Toughness' is the *total* area under the curve up to fracture (elastic + plastic), not just the elastic part.
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