The micro - organisms which can reproduce and multiply only inside the cells of other organisms are :

Options

A. Protozoa
B. fungi
C. bacteria
D. viruses

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about microorganisms that can reproduce and multiply only inside the cells of other organisms.

2. **Analyzing the Options**: We need to evaluate the given options to identify which microorganism fits this description. The options provided are protozoa, fungi, bacteria, and viruses.

3. **Evaluating Protozoa**: Protozoa, such as Plasmodium, can reproduce both inside a host organism (like in the case of malaria) and outside in water. Therefore, they do not exclusively multiply inside other cells.

4. **Evaluating Fungi**: Most fungi thrive in moist environments and can reproduce independently outside of host cells. They do not require the cells of other organisms for reproduction.

5. **Evaluating Bacteria**: Bacteria can grow and reproduce outside of host organisms. They can divide and multiply independently in various environments.

6. **Evaluating Viruses**: Viruses are unique in that they cannot reproduce on their own. They require a host cell to replicate. Once inside a host cell, they hijack the cell's machinery to reproduce and multiply.

7. **Conclusion**: Based on the evaluations, the only microorganisms that can reproduce and multiply exclusively inside the cells of other organisms are viruses.

**Final Answer**: Viruses

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