b
$\left|\begin{array}{ccc}{a} & {-a^{2}} & {a^{3}} \\ {-a^{2}} & {a^{3}} & {a} \\ {a^{3}} & {a} & {-a^{2}}\end{array}\right|=0$
$\Rightarrow a^{3}(a+1)^{2}\left(a^{2}-a+1\right)^{2}=0$
$ \Rightarrow {\rm{a}} = 0, - 1, - \omega , - {\omega ^2}$
$\Rightarrow \mathrm{a}=-\omega,-\omega^{2}$ (non real)