The correct option is (a) Constant-strain triangle

Easiest explanation: In a linear triangular element (number of nodes=3), there are two degrees of freedom, ux and uy per node and a total of six nodal displacements per element. Since the first derivatives of shape functions for a triangular element are element-wise constant, all the strains computed for the linear triangular element are element-wise constant. Therefore, the linear triangular element for a plane elasticity problem is known as the constant-strain triangular (CST) element.