When a force is applied at one end of an elastic wire, it produces a strain ϵ in the wire. If Y is Young's modulus of the material of the wire, the amount of energy stored per unit volume of the wire is given by
Yϵ22
Energy stored per unit volume =12 (stress × strain).
But stress = Young's modulus × strain = Yϵ
Hence, energy stored per unit volume =Yϵ22, which is choice (d).