C

- Hướng dẫn giải

Ta có:

\(\begin{array}{l}
\frac{1}{{2 + \sqrt 3 }} + \frac{1}{{2 - \sqrt 3 }} = \frac{{2 - \sqrt 3 }}{{\left( {2 + \sqrt 3 } \right).\left( {2 - \sqrt 3 } \right)}} + \frac{{2 + \sqrt 3 }}{{\left( {2 - \sqrt 3 } \right).\left( {2 + \sqrt 3 } \right)}}\\
 = 2 - \sqrt 3  + 2 + \sqrt 3  = 4
\end{array}\)