Symbol `D` stands for:

Options

A. (a) Dextrorotatory, which rotates `P.P.L.` towards right.
B. (b) Dextrorotatory, which rotates `P.P.L`. towards left.
C. (c) Relative configuration with respect to lactic acid taken as standard.
D. (d) Relative configuration with respect to glyceraldehyde taken as standard and `(OH)` group is on the right side.

Answer

To determine what the symbol `D` stands for in the context of isomerism, particularly in organic chemistry, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Optical Activity**:
- Optical activity refers to the ability of a compound to rotate plane-polarized light. Compounds can be classified as dextrorotatory (rotating light to the right) or levorotatory (rotating light to the left).
- The symbols used for these classifications are small `d` for dextrorotatory and small `l` for levorotatory.

**Hint**: Recall that dextrorotatory compounds are those that rotate light to the right, while levorotatory compounds rotate it to the left.

2. **Identifying Capital D and L**:
- The capital letters `D` and `L` do not refer to optical activity but rather to the relative configuration of certain sugars and amino acids.
- The designation of `D` or `L` is based on the configuration of the molecule in relation to the reference compound glyceraldehyde.

**Hint**: Remember that `D` and `L` are related to the configuration of molecules and not their optical activity.

3. **Reference Compound**:
- Glyceraldehyde is the reference compound used to determine whether a sugar or amino acid is classified as `D` or `L`.
- If the hydroxyl group (OH) on the penultimate carbon (the second to last carbon) of the molecule is on the right side in a Fischer projection, the compound is designated as `D`. If it is on the left side, it is designated as `L`.

**Hint**: Visualize the Fischer projection of glyceraldehyde to understand the positioning of the hydroxyl group.

4. **Conclusion**:
- Therefore, the symbol `D` stands for the relative configuration of a molecule with respect to glyceraldehyde, specifically indicating that the hydroxyl group is on the right side.

**Hint**: Think of `D` as indicating the right-hand position of the hydroxyl group in the Fischer projection of glyceraldehyde.

### Final Answer:
The symbol `D` stands for relative configuration with respect to glyceraldehyde, where the hydroxyl group is on the right side.