A monosaccharide is

Options

A. Lactose
B. Sucrose
C. Ribose
D. Maltose.

Answer

To determine which of the given options is a monosaccharide, we need to understand what a monosaccharide is and how it differs from disaccharides and polysaccharides.

### Step-by-Step Solution:

1. **Definition of Monosaccharide**:
- A monosaccharide is the simplest form of carbohydrate. It consists of a single sugar unit and cannot be hydrolyzed into simpler sugars.

2. **Chemical Composition**:
- Monosaccharides have the general chemical formula of \( C_nH_{2n}O_n \), where \( n \) is typically a number between 3 and 7. They contain carbon (C), hydrogen (H), and oxygen (O) in a specific ratio.

3. **Examples of Carbohydrates**:
- The options provided are lactose, sucrose, ribose, and maltose.
- **Lactose**: A disaccharide made up of glucose and galactose.
- **Sucrose**: A disaccharide made up of glucose and fructose.
- **Maltose**: A disaccharide made up of two glucose units.
- **Ribose**: A monosaccharide with the molecular formula \( C_5H_{10}O_5 \).

4. **Identifying the Monosaccharide**:
- Among the options, ribose is the only one that is a monosaccharide. The other three (lactose, sucrose, and maltose) are all disaccharides, as they are composed of two monosaccharide units.

5. **Conclusion**:
- Therefore, the correct answer is **ribose**, as it is a monosaccharide, while lactose, sucrose, and maltose are disaccharides.

### Final Answer:
The correct option is **ribose**.