c
Let $A = \left( {\begin{array}{*{20}{c}}
{\alpha - 1}\\
0\\
0
\end{array}} \right),B = \left( {\begin{array}{*{20}{c}}
{\alpha + 1}\\
0\\
0
\end{array}} \right)$
be two matrices.
$A{B^T} = \left( {\begin{array}{*{20}{c}}
{\alpha - 1}\\
0\\
0
\end{array}} \right)\left( {\begin{array}{*{20}{c}}
{\alpha + 1}&0&0
\end{array}} \right)$
$ = \left( {\begin{array}{*{20}{c}}
{{\alpha ^2} - 1}&0&0\\
0&0&0\\
0&0&0
\end{array}} \right)$
Thus, $A{B^T}$ is non-zero matrix for $\left| \alpha \right| \ne 1$