The simplest unicellular organisms having rigid cell wall like plant cells are:

Options

A. virus
B. bacteria
C. protozoans
D. fungi

Explanation

To solve the question "The simplest unicellular organisms having rigid cell wall like plant cells are:", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for the simplest unicellular organisms that have a rigid cell wall similar to that of plant cells.

2. **Identifying the Options**: The options provided are:
- Virus
- Bacteria
- Protozoans
- Fungi

3. **Analyzing Each Option**:
- **Virus**: Viruses are not considered unicellular organisms as they are acellular (not made of cells) and require a host to replicate. Therefore, they do not have a cell wall.
- **Protozoans**: Protozoans are unicellular eukaryotic organisms, but they do not have a rigid cell wall. Instead, they have flexible cell membranes.
- **Fungi**: While some fungi can be unicellular (like yeast), they are primarily multicellular. Additionally, their cell walls are made of chitin, not cellulose like in plants.
- **Bacteria**: Bacteria are unicellular prokaryotic organisms. They have a rigid cell wall, which is primarily composed of peptidoglycan. This cell wall provides structural support and protection.

4. **Conclusion**: Based on the analysis, the only option that fits the criteria of being a unicellular organism with a rigid cell wall is **Bacteria**.

### Final Answer:
The simplest unicellular organisms having rigid cell walls like plant cells are **Bacteria**.

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