Which among the following is not a blood cell?

Options

A. Serum
B. Erythrocytes
C. Leucocytes
D. Thrombocytes

Explanation

To determine which among the following is not a blood cell, we need to analyze the given options:

1. **Serum**
2. **Erythrocytes**
3. **Leukocytes**
4. **Thrombocytes**

### Step-by-Step Solution:

**Step 1: Identify Blood Cells**
- Blood cells are classified into three main types:
- **Erythrocytes** (Red Blood Cells or RBCs): Responsible for transporting oxygen.
- **Leukocytes** (White Blood Cells or WBCs): Part of the immune system, helping to fight infections.
- **Thrombocytes** (Platelets): Involved in blood clotting.

**Step 2: Analyze Each Option**
- **Erythrocytes**: This is a type of blood cell (RBC).
- **Leukocytes**: This is also a type of blood cell (WBC).
- **Thrombocytes**: These are platelets, which are considered blood cells involved in clotting.
- **Serum**: This is the liquid part of blood that remains after clotting has occurred. It does not contain blood cells.

**Step 3: Conclusion**
- Since serum does not contain any blood cells (RBCs, WBCs, or platelets), it is the correct answer to the question.

### Final Answer:
**Serum is not a blood cell.**