Which one is the smallest organism capable of autonomous growth and reproduction
Or
Which among the following are the smallest living cells, known without a definite cell wall, pathogenic to plants as well as animals and can survive without oxygen

Options

A. Bacillus
B. Pseudomonas
C. Mycoplasma
D. Nostoc.

Answer

To answer the question regarding the smallest organism capable of autonomous growth and reproduction, or the smallest living cells that lack a definite cell wall, are pathogenic to plants and animals, and can survive without oxygen, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Characteristics:**
- The question asks for the smallest organism capable of autonomous growth and reproduction.
- It also mentions organisms that are the smallest living cells, lack a definite cell wall, are pathogenic to both plants and animals, and can survive without oxygen.

2. **Consider the Options:**
- The organisms that fit the description of being the smallest and capable of autonomous growth and reproduction are typically bacteria and certain types of viruses.
- However, viruses are not considered living organisms as they cannot reproduce independently without a host.

3. **Focus on Prokaryotes:**
- Among prokaryotes, the smallest known organisms are certain types of bacteria.
- Mycoplasma is a genus of bacteria that is known for lacking a cell wall, making them the smallest free-living cells.

4. **Pathogenic Nature:**
- Mycoplasma species can be pathogenic to both plants and animals. For example, Mycoplasma pneumoniae is known to cause respiratory infections in humans.

5. **Anaerobic Survival:**
- Mycoplasma can survive in environments without oxygen, which aligns with the question's criteria.

6. **Conclusion:**
- Therefore, the answer to the question is **Mycoplasma**. They are the smallest living cells, lack a definite cell wall, can be pathogenic, and can survive without oxygen.

### Final Answer:
**Mycoplasma**