Correct option is (b) Lagrange family interpolation function

To explain I would say: An examination of expanded weak forms of the plane elasticity problems reveals that the variables ux and uy are the primary variables, and only first derivatives of ux and uy with respect to x and y appear, respectively. Therefore, ux and uy must be approximated by the Lagrange family of interpolation functions, and at least bilinear (i.e., linear both in x and y) interpolation is required.