The correct answer is option 2. Configuration.
The letters 'D' and 'L' in the context of carbohydrates refer to the configuration of the asymmetric carbon farthest from the carbonyl group in aldoses or the functional group in ketoses. This asymmetric carbon is also known as the chiral center or chiral carbon.
Let us break down the explanation:
1. Chirality in Carbohydrates:
Carbohydrates often have multiple chiral centers (carbon atoms bonded to four different groups). These chiral centers give rise to stereoisomers, specifically enantiomers, which are mirror images of each other.
2. D and L Configuration:
- D Configuration: In the Fischer projection of sugar, if the hydroxyl group on the highest-numbered chiral carbon is on the right side, it is designated as the D form.
- L Configuration: If the hydroxyl group on the highest-numbered chiral carbon is on the left side, it is designated as the L form.
3. Highest-Numbered Chiral Carbon:
The highest-numbered chiral carbon is determined by numbering the carbon atoms in the sugar chain, starting with the carbon closest to the carbonyl group in aldoses or the functional group in ketoses.
4. Optical Activity:
The D and L configuration is related to the optical activity of the molecule.
D-sugars are often referred to as dextrorotatory, meaning they rotate plane-polarized light clockwise.
L-sugars are levorotatory, meaning they rotate plane-polarized light counterclockwise.
5. Biological Significance:
In nature, many common sugars found in living organisms are of the D-configuration. For example, D-glucose is a common sugar in biological systems.
In summary, the letters 'D' and 'L' indicate the spatial arrangement of substituents around the highest-numbered chiral carbon in carbohydrates. The configuration is determined based on the direction of rotation of plane-polarized light and is important in understanding the stereochemistry of sugars in biological systems. | 
