The number of stereoisomers formed in a reaction of $(\pm) Ph ( C = O ) C ( OH )( CN ) Ph$ with $HCN$ is $..........$

$2$-chlorobutane $+\mathrm{Cl}_2 \rightarrow \mathrm{C}_4 \mathrm{H}_8 \mathrm{Cl}_2$ (isomers)

Total number of optically active isomers shown by $\mathrm{C}_4 \mathrm{H}_8 \mathrm{Cl}_2$, obtained in the above reaction is......................

Number of diastereomer of given compound

The number of meso form of the given compound is

$\begin{array}{*{20}{c}}
  {C{H_3} - CH - CH - CH - C{H_3}} \\ 
  {|\,\,\,\,\,\,\,\,\,\,\,\,\,\,|\,\,\,\,\,\,\,\,\,\,\,\,|\,\,\,\,\,} \\ 
  {OH\,\,\,\,\,\,\,OH\,\,\,\,\,OH\,\,} 
\end{array}$

How many number of optically active isomers are possible for $2,3-$ dibromo butane $1,4-$ dioic acid ?

Which of the following reactions will not produce a racemic product ?