Answer
D and L designation applies only for chiral molecules. When glyceraldehyde is reduced to glycerol, there are no chiral carbons in the molecule, so D and L isomer designation is not applicable.
Work Step by Step
When glyceraldehyde is reduced to glycerol, C1 and C3 have the same atoms and stereochemistry, so C2 is not chiral anymore. This means D and L designation is not applicable to glycerol.
