Final answer:
The 'D' sign represents the configuration of chiral compounds in the D series, where the reference group is on the right side of the last chiral center from the functional group.
Step-by-step explanation:
The 'D' sign in the context mentioned refers to the configuration of chiral compounds. It indicates that the compound belongs to the D series. In organic chemistry, the D series compounds are those that contain the reference group on the right side of the last chiral center from the functional group.
For example, D-glucose has the -OH group on the right side of the fifth carbon atom in its straight chain structure.