\(\begin{array}{*{20}{c}}
  {\,\,\,\,\,\,\,\,\,COOH} \\ 
  {|\,\,\,\,} \\ 
  {H - C - OH} \\ 
  { -  -  - | -  -  -  - } \\ 
  {H - C - OH} \\ 
  {|\,\,\,\,} \\ 
  {\,\,\,\,\,\,\,\,\,COOH} 
\end{array}\)

Plane of Symmetry

Mesotartaric acid is optically inactive due to internal compensation i.e. the effect one half of the molecule is neutralized by other.