(c)
Given, $\left| \overrightarrow{\mathbf{a}} \times \overrightarrow{\mathbf{b}} \right| = \left| \overrightarrow{\mathbf{a}} \bullet \overrightarrow{\mathbf{b}} \right|$
$\Longrightarrow \left| \overrightarrow{\mathbf{a}} \right| \bullet \left| \overrightarrow{\mathbf{b}} \right|\sin{\theta =}\left| \overrightarrow{\mathbf{a}} \right|\left| \overrightarrow{\mathbf{b}} \right|\cos\theta$
$\Longrightarrow \sin{\theta = \cos{\theta\ \Longrightarrow \theta = \frac{\pi}{4}}}$