The total number of optically active and meso forms possible for the compound
$HOOC – CHOH-CHOH-CHOH-COOH$ respectively are ………………..

The molecule is symmetrical and has 3 chiral carbon atons ((two real and one pseudo)
               Number of stereo isomers $=2^{3-1}=4$
               Number of meso-forms=$2^{\frac{3}{2}-\frac{1}{2}}$ =2
               No. of optical isomers =4-2=2