\(\begin{array}{l}
21)\,y' = 3{x^2} - 2mx + {m^2} - 6\\
\Delta ' = {m^2} - 3\left( {{m^2} - 6} \right) = 18 - 2{m^2}\\
ycbt \Rightarrow \Delta ' > 0 \Leftrightarrow 18 - 2{m^2} > 0\\
\Leftrightarrow {m^2} < 9 \Rightarrow - 3 < m < 3 \Rightarrow m \in \left\{ { - 2; - 1;0;1;2} \right\}\\
\Rightarrow A
\end{array}\)