Which of the following carbohydrate is a reducing sugar ?

Options

A. Maltose
B. Glucose
C. Galactose
D. All of above

Answer

To determine which of the given carbohydrates is a reducing sugar, we need to analyze each option based on the presence of a free anomeric carbon. A reducing sugar is defined as a sugar that has a free aldehyde or ketone group, which can reduce other substances.

### Step-by-Step Solution:

1. **Identify the Options**: The options given are maltose, glucose, galactose, and all of the above.

2. **Understanding Reducing Sugars**: A reducing sugar must have a free anomeric carbon. This means that the hydroxyl group (-OH) on the anomeric carbon must be available to participate in reduction reactions.

3. **Analyzing Glucose**:
- Glucose is a monosaccharide with the structure that includes an anomeric carbon (C1).
- The anomeric carbon in glucose has a free -OH group.
- Therefore, glucose is a reducing sugar.

4. **Analyzing Galactose**:
- Galactose is also a monosaccharide and is an epimer of glucose.
- It also has an anomeric carbon (C1) with a free -OH group.
- Thus, galactose is also a reducing sugar.

5. **Analyzing Maltose**:
- Maltose is a disaccharide composed of two glucose units.
- In maltose, the first glucose unit's anomeric carbon (C1) is involved in a glycosidic bond with the second glucose unit (C4).
- However, the anomeric carbon of the second glucose unit remains free and has a -OH group.
- Therefore, maltose is also a reducing sugar.

6. **Conclusion**: Since all three carbohydrates (glucose, galactose, and maltose) have free anomeric carbons with -OH groups, they are all reducing sugars.

### Final Answer:
The correct option is **D. All of the above**.