Answer: D
\((\cos A + \cos C)\) \(= \frac{AB}{AC} + \frac{BC}{AC}\) \(= \frac{AB + BC}{AC}\) \(= \frac{8}{\sqrt{15}}\) So, the value of \((\cos A + \cos C)\) is \(\frac{8}{\sqrt{15}}\).
\((\cos A + \cos C)\) \(= \frac{AB}{AC} + \frac{BC}{AC}\) \(= \frac{AB + BC}{AC}\) \(= \frac{8}{\sqrt{15}}\) So, the value of \((\cos A + \cos C)\) is \(\frac{8}{\sqrt{15}}\).