Step-by-step explanation:
Meso compounds are optically inactive stereoisomers. Despite of having chiral carbon, they do not show optical activity because it has a plane of symmetry in its structure itself. It is superposable on its mirror image.
Aldo sugars on reaction which reducing agents such as , NaBH₄ reduces the carbonyl group in the sugar to alcohol and gives corresponding alditol.
The D- aldohexose which on reduction gives a meso alditol are allose and galactose.
The structure is shown in the image below. Thus, in allitol and galactitol formed have internal plane of symmetry which makes the optically inactive.