If \(\tan X = \frac{m}{n}\) and 0° ≤ X ≤ 90°, then the value of (sin X + cos X) is:

• A. \(\frac{1}{{\sqrt {{m^2} + {n^2}} }}\)
• B. \(\sqrt {{m^2} - \;{n^2}} \)
• C. \(\frac{1}{{\sqrt {{m^2}\; - \;{n^2}} }}\)
• D. \(\frac{{m + n}}{{\sqrt {{m^2}\; + {n^2}\;} }}\)