Explanation

(c)
The projection of
$\overrightarrow{\mathbf{a}}\ on\ \overrightarrow{\mathbf{b}} = \frac{\overrightarrow{\mathbf{a}}\ \bullet \overrightarrow{\mathbf{b}}}{\left| \overrightarrow{\mathbf{b}} \right|}$
$= \frac{\left( 3\widehat{\dot{\mathbf{i}}} - \widehat{\dot{\mathbf{j}}} + 5\widehat{\mathbf{k}} \right) \bullet (2\widehat{\dot{\mathbf{i}}} + 3\widehat{\dot{\mathbf{j}}} + \widehat{\mathbf{k}})}{\sqrt{2^{2} + 3^{2} + 1^{2}}} = \frac{8}{\sqrt{14}}$