c
(c) The system of homogeneous equations
$x - cy - bz = 0$
$cx - y + az = 0$
$bx + ay - z = 0$
has a non-trivial solution (since $x,\,y,\,z$ are not all zero)
If $\Delta = \left| {\,\begin{array}{*{20}{c}}1&{ - c}&{ - b}\\c&{ - 1}&a\\b&a&{ - 1}\end{array}\,} \right|\, = 0$
i.e., if $(1 - {a^2})\, + c( - c - ab) - b(ac + b) = 0$
i.e., if ${a^2} + {b^2} + {c^2} + 2abc = 1$.