(a)
Since, $\overrightarrow{\mathbf{b}} \times \overrightarrow{\mathbf{c}}$ is a vector perpendicular to $\overrightarrow{\mathbf{b}},\overrightarrow{\mathbf{c}}.$ Therefore $\overrightarrow{\mathbf{a}} \times (\overrightarrow{\mathbf{b}} \times \overrightarrow{\mathbf{c}})$ is a vector again in plane of $\overrightarrow{\mathbf{b}},\overrightarrow{\mathbf{c}}.$