The correct option is (a) Body force

Best explanation: Of the four terms present in the vector form of the principle of virtual displacements applied to plane finite elastic element, 0=\(\int_{V_e}\)(σijδεij+ρüiδui)dV-\(\int_{V_e}\)fiδuidV-∮set̂iδuids, the first term one corresponds to the virtual strain energy stored in the body. The second term corresponds to the kinetic energy stored in the body. The given term is the third term in the equation, and that represents the virtual work done by the body force.