The organisms which lack a cell wall and can live without oxygen are

Options

A. Thermoacidophiles
B. Methanogens
C. Archaebacteria
D. Mycoplasma

Answer

To answer the question "The organisms which lack a cell wall and can live without oxygen are," we need to analyze the options provided and understand the characteristics of each group.

### Step-by-Step Solution:

1. **Identify the Options**: The options given are:
- Thermoacetophiles
- Methanogens
- Arc bacteria
- Mycoplasma

2. **Understand Thermoacetophiles**:
- Thermoacetophiles are a type of archaea that thrive in high temperatures and acidic environments.
- They do have a cell wall, so they do not fit the criteria of lacking a cell wall.

3. **Understand Methanogens**:
- Methanogens are also a type of archaea that produce methane and can survive in anaerobic (without oxygen) conditions.
- However, they typically have a cell wall, which disqualifies them from the answer.

4. **Understand Arc Bacteria**:
- Arc bacteria, or archaea, are known for surviving in extreme conditions.
- While some can live without oxygen, they generally have a cell wall, so they do not meet the criteria.

5. **Understand Mycoplasma**:
- Mycoplasma are a unique group of bacteria that lack a cell wall entirely.
- They are known for their ability to survive without oxygen (anaerobic) and are highly pleomorphic, meaning they can change shape.
- They are often referred to as the smallest cells and are known as "PPLO" or "pleuropneumonia-like organisms."

6. **Conclusion**:
- Based on the characteristics analyzed, the correct answer to the question is **Mycoplasma**, as they are the only organisms listed that lack a cell wall and can live without oxygen.

### Final Answer:
The organisms which lack a cell wall and can live without oxygen are **Mycoplasma**.

---