Right answer is (a) \(\begin{pmatrix}\bar{c}_{11} & \bar{c}_{12} & 0 \\\bar{c}_{12} & \bar{c}_{22} & 0 \\0 & 0 & \bar{c}_{66} \end{pmatrix}\)

The explanation: For an orthotropic material under plane strain, with principal material axes (x1, x2, x3) coinciding with the (x, y, z) coordinates, the relation between stress and strain components is \(\begin{pmatrix}\sigma_{xx} \\ \sigma_{yy} \\ \sigma_{xy}\end{pmatrix} = \begin{pmatrix}\bar{c}_{11} & \bar{c}_{12} & 0 \\\bar{c}_{12} & \bar{c}_{22} & 0 \\0 & 0 & \bar{c}_{66} \end{pmatrix} = \begin{pmatrix}\varepsilon_{xx} \\ \varepsilon_{yy} \\ 2 \varepsilon_{xy}\end{pmatrix}\) where C is the elastic stiffness matrix. The state of stress is σxz=σyz=0 and \(\sigma_{zz}=E_3(\frac{v_{13}}{E_1}\sigma_{xx}+\frac{v_{23}}{E_2}\sigma_{yy})\).