Monosaccharides are carbohydrates which cannot be hydrolyzed to simple molecules. They may contain 3-7 carbon atoms but monosaccharides containing five and six carbon atoms are more abundant in nature. Those containing an aldehyde group are called aldoses while those containing a keto group are called ketoses. The aldehyde group is always present at C₁ while the keto group is usually present at C₂.  All monosaccharides reduce Tollens reagent as well as Fehling's solution and hence are called reducing sugars. Pentoses ands hexoses have cyclic structures, furanose (five membered) and pyranose (six membered). Ribose in RNA and fructose in sucrose exist in the furanose form while glucose, mannose, galactose, etc. all exist in the pyranose form. During ring formation C₁ in aldohexoses and C₂ in fructose becomes chiral and hence all these monosaccharides exist in two stereoisomeric forms called the  α-anomer and the β-anomer while C₁ and C₂ are called glycosidic or anomeric carbon atoms and the bonds connected to glycosidic carbon are called glycosidic linkages. In contrast, stereoisomers, which differ in configuration at any other chiral carbon are called epimers. When two molecule of the same or different monosaccharide combine together through glycosidic linkage, a disaccharide is formed. All monosaccharides and reducing disaccharides react with excess of phenyl hydrazine to form osazones which are oftenly used for their characterization. 

Two forms of D-glucopyranose are called 

Anomers 

The correct answer is option 2. Anomers.

Stereoisomers are molecules with the same molecular formula and sequence of bonded atoms (constitution), but different three-dimensional orientations.

1. Enantiomers:

Enantiomers are a type of stereoisomer. They are non-superimposable mirror images of each other. Example: D-glucose and L-glucose.

3. Epimers:

Epimers are stereoisomers that differ in configuration at exactly one chiral carbon other than the anomeric carbon. Example: D-glucose and D-mannose differ at C-2.

4. Diastereomers:

Diastereomers are stereoisomers that are not mirror images of each other. They differ at one or more (but not all) chiral centers. Example: D-glucose and D-galactose differ at C-4.

2. Anomers

Anomers are a specific type of epimer. They differ specifically at the anomeric carbon, which is the carbon derived from the carbonyl group (aldehyde or ketone) during the cyclization of a sugar molecule. When D-glucose cyclizes to form a six-membered ring (glucopyranose), the following process occurs:

Formation of the Hemiacetal:

The aldehyde group (at C-1) reacts with the hydroxyl group (at C-5) to form a hemiacetal. This creates a new chiral center at C-1, known as the anomeric carbon.

Two Possible Configurations:

α-Anomer: The hydroxyl group attached to the anomeric carbon (C-1) is on the opposite side (trans) to the CH₂OH group attached to C-5.

β-Anomer: The hydroxyl group attached to the anomeric carbon (C-1) is on the same side (cis) as the CH₂OH group attached to C-5.

Conclusion

The α and β forms of D-glucopyranose are anomers because they differ in the configuration around the anomeric carbon (C-1).

Summary

Anomers differ only at the anomeric carbon. The anomeric carbon is created during the cyclization of the sugar molecule. In D-glucopyranose, the α-anomer has the hydroxyl group at C-1 trans to the CH₂OH group at C-5, while the β-anomer has the hydroxyl group at C-1 cis to the CH₂OH group at C-5.

Thus, the two forms of D-glucopyranose are correctly identified as: Anomers