Haemoglobin is specifically described as a :

Options

A. protein
B. lipoprotein
C. enzyme
D. nucleolipid

Explanation

### Step-by-Step Solution:

1. **Understanding Hemoglobin**:
- Hemoglobin is a crucial component of the blood, specifically found in red blood cells (erythrocytes). It plays a vital role in the respiratory system by facilitating the transport of oxygen from the lungs to the body's tissues.

2. **Identifying the Nature of Hemoglobin**:
- Hemoglobin is often referred to as a "respiratory pigment" because it binds with oxygen. This binding is reversible, allowing hemoglobin to pick up oxygen in the lungs and release it in tissues where it is needed.

3. **Chemical Composition**:
- Hemoglobin is composed of four subunits, which are globular proteins. Each subunit contains an iron atom that binds to oxygen, giving hemoglobin its red color and its ability to transport oxygen.

4. **Eliminating Incorrect Options**:
- **Lipoprotein**: This is a complex of lipids and proteins. Hemoglobin does not contain lipids, so this option is incorrect.
- **Enzyme**: Enzymes are biological catalysts that speed up chemical reactions. Hemoglobin does not catalyze reactions; it primarily functions in gas transport, making this option incorrect.
- **Nucleolipid**: This term refers to a combination of nucleic acids and lipids. Hemoglobin does not fit this description, so this option is also incorrect.

5. **Conclusion**:
- Based on the above analysis, hemoglobin is specifically described as a **protein**. Therefore, the correct answer is **option A: protein**.