d
Maleic acid and fumaric acids are geometrical isomers. 

\(\mathop {\begin{array}{*{20}{c}}
  {H - C - COOH} \\ 
  {||\,\,\,\,\,\,\,\,\,\,\,} \\ 
  {H - C - COOH} 
\end{array}}\limits_{Maleic\,acid} \) \(\mathop {\begin{array}{*{20}{c}}
  {H - C - COOH} \\ 
  {||\,\,\,\,\,\,\,\,\,\,\,\,\,} \\ 
  {HOOC - C - H\,\,\,\,\,\,} 
\end{array}}\limits_{Fumaric\,acid} \)