An obligate anaerobe is

Options

A. Ulothrix
B. Spirogyra
C. Methane bacteria
D. Onion.

Answer

### Step-by-Step Solution:

1. **Understanding Anaerobes**:
- Anaerobes are organisms that can survive in environments devoid of oxygen. They thrive in anaerobic conditions.

2. **Types of Anaerobes**:
- Anaerobes can be classified into three categories:
- **Obligate Anaerobes**: These organisms require a complete absence of oxygen to survive. Exposure to oxygen is harmful or lethal to them.
- **Aerotolerant Anaerobes**: These can tolerate oxygen but do not use it for growth. They can survive in its presence but do not require it.
- **Facultative Anaerobes**: These can grow in both the presence and absence of oxygen. They can switch between aerobic and anaerobic metabolism.

3. **Analyzing the Options**:
- **Eulothrix**: This is a filamentous green alga, which is not an anaerobe. Thus, it is not the correct answer.
- **Spirogyra**: Another filamentous green alga, which also does not fit the definition of an obligate anaerobe. Hence, it is incorrect.
- **Methane Bacteria**: These are known as methanogens and are obligate anaerobes. They thrive in environments without oxygen and are essential in processes like anaerobic digestion.
- **Onion**: This is a vegetable and does not fall under the category of anaerobes. Therefore, it is also incorrect.

4. **Conclusion**:
- The only option that qualifies as an obligate anaerobe is **Methane Bacteria**.

### Final Answer:
**Methane Bacteria** is the correct answer as it is an obligate anaerobe.

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