(c) $\overrightarrow{A} \bullet \overrightarrow{B} = ABcos\theta = 6$
and $\left| \overrightarrow{A} \times \overrightarrow{B} \right| = ABsin\theta = 6\sqrt{3}$
$\therefore\ \frac{ABsin\theta}{ABcos\theta} = \frac{6\sqrt{3}}{6} = \sqrt{3}$
or tan$\theta = \sqrt{3}$
and $\theta = 60{^\circ}$