Right answer is (c) Linear triangular and linear quadrilateral elements

For explanation I would say: Because the first-derivatives of the primary variables, ux and uy with respect to x and y, respectively, appear in the expanded weak forms of the plane elasticity problems, they must be approximated by the Lagrange family of interpolation functions, with at least a bilinear interpolation. The simplest elements that satisfy those requirements are the linear triangular and linear quadrilateral elements.