In FEM, which method is not used to construct the weak forms and associated finite element model of the plane elasticity equations?

(a) Principle of virtual displacements

(b) The principle of minimum total potential energy

(c) Weak form of governing differential equations

(d) Hamiltonian principle

The question was asked in my homework.

My question is based upon Plane Elasticity topic in section Plane Elasticity of Finite Element Method