**D-Glucose Stereoisomers**

D-Glucose is a monosaccharide, a type of carbohydrate, and it exists in different forms known as stereoisomers. Stereoisomers are compounds that have the same molecular formula and connectivity but differ in the spatial arrangement of their atoms. In the case of D-Glucose, the arrangement of its hydroxyl groups around the asymmetric carbon atoms gives rise to different stereoisomers.

**Definition of Stereoisomers**

Stereoisomers can be categorized into two main types: enantiomers and diastereomers.

1. **Enantiomers**: Enantiomers are a pair of stereoisomers that are non-superimposable mirror images of each other. They have the same connectivity but differ in their three-dimensional orientation. Enantiomers are optically active, meaning they rotate plane-polarized light in opposite directions. In the case of D-Glucose, there are two enantiomers: D-Glucose and L-Glucose.

2. **Diastereomers**: Diastereomers are stereoisomers that are not mirror images of each other. They have different connectivity or spatial arrangement at one or more stereocenters. Diastereomers are not optically active. In the case of D-Glucose, there are three diastereomers: α-D-Glucose, β-D-Glucose, and open-chain D-Glucose.

**Explanation of the Answer: 3**

The question asks for the number of different forms (stereoisomers) in which D-Glucose exists. The correct answer is '3'. Here's the breakdown of the three stereoisomers:

1. **Enantiomers (1 stereoisomer)**:
- D-Glucose (Dextrose): It is the naturally occurring form of glucose found in living organisms.
- L-Glucose: It is the mirror image of D-Glucose and is not commonly found in nature.

2. **Diastereomers (2 stereoisomers)**:
- α-D-Glucose: It is a cyclic form of glucose where the hydroxyl group on the anomeric carbon is in the axial position.
- β-D-Glucose: It is another cyclic form of glucose where the hydroxyl group on the anomeric carbon is in the equatorial position.

Therefore, considering both enantiomers and diastereomers, D-Glucose exists in a total of 3 different forms or stereoisomers.