Which is a reducing sugar?

Options

A. Galactose
B. Gluconic acid
C. `beta`-methyl galactoside
D. Sucrose

Answer

To determine which is a reducing sugar, we need to understand the characteristics of reducing sugars and analyze the provided options. Here’s a step-by-step solution:

### Step 1: Define Reducing Sugar
A reducing sugar is a type of sugar that can act as a reducing agent. This means it can donate electrons to another molecule, which typically results in the formation of a carboxylic acid or a ketone when oxidized.

### Step 2: Identify Common Reducing Sugars
Common examples of reducing sugars include monosaccharides like glucose, galactose, and fructose, as well as some disaccharides such as lactose and maltose. Sucrose, however, is generally not considered a reducing sugar because it does not have a free aldehyde or ketone group.

### Step 3: Analyze the Options
- **Galactose**: This is a monosaccharide and is a reducing sugar.
- **Gluconic Acid**: This is an oxidized form of glucose and is not classified as a sugar.
- **Sucrose**: This is a disaccharide made up of glucose and fructose, but it does not have a free aldehyde or ketone group, making it a non-reducing sugar.
- **Lactose**: This is a disaccharide made up of glucose and galactose and is a reducing sugar.

### Step 4: Conclusion
From the analysis, the reducing sugars among the options are galactose and lactose. However, since the question asks for a single reducing sugar, we can conclude that **galactose** is a reducing sugar.

### Final Answer
**Galactose is a reducing sugar.**

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