(c)
$Area\ of\ rhombus = \frac{1}{2}\left| \overrightarrow{\mathbf{a}} \times \overrightarrow{\mathbf{b}} \right|$
$= \frac{1}{2}\left| \left( 2\widehat{\dot{\mathbf{i}}} - 3\widehat{\dot{\mathbf{j}}} + 5\widehat{\mathbf{k}} \right) \times \left( - \widehat{\dot{\mathbf{i}}} + \widehat{\dot{\mathbf{j}}} + \widehat{\mathbf{k}} \right) \right|$
$= \frac{1}{2}\left| - 8\widehat{\dot{\mathbf{i}}} - 7\widehat{\dot{\mathbf{j}}} - \widehat{\mathbf{k}} \right| = \frac{1}{2}\sqrt{144}$
$= \sqrt{28.5}$