(c)
Given,
$\left( \overrightarrow{\mathbf{a}} \times \overrightarrow{\mathbf{b}} \right) \bullet \left( \overrightarrow{\mathbf{a}} \times \overrightarrow{\mathbf{b}} \right) = \frac{1}{4}$
$\Longrightarrow (\left| \overrightarrow{\mathbf{a}} \right|\left| \overrightarrow{\mathbf{b}} \right|\sin{\theta)}^{2} = \frac{1}{4}$
$\Longrightarrow \ \sin^{2}\theta = \frac{1}{4}$
$\Longrightarrow \ \theta = \frac{\pi}{6}$