Right choice is (b) u0=\(\frac{1}{2}\)E(ε-ε0)
The best I can explain: Energy stored in a body due to deformation is called Strain energy. The strain energy per unit volume is called strain energy density and the area under the stress strain curve towards the point is deformation. Strain energy per unit volume is
u0=\(\frac{1}{2}\)E(ε-ε0)