Smallest bacterium is

Options

A. Dialister
B. Nitrosomonas
C. Bacillus
D. Spirillum.

Answer

**Step-by-Step Solution:**

1. **Identify the Question:** The question asks for the smallest bacterium among the given options.

2. **List the Options:** The options provided are:
- Option 1: Dallister
- Option 2: Nitrosomonas
- Option 3: Bacillus
- Option 4: Spirulina

3. **Analyze Each Option:**
- **Dallister:** This is known to be the smallest bacterium. Its species is Pneumocinus, which is an anaerobic, gram-negative bacillus. The size of Dallister is approximately 0.5 x 0.5 x 1.6 micrometers.
- **Nitrosomonas:** This bacterium is involved in nitrogen fixation but is not the smallest.
- **Bacillus:** This is a genus of bacteria that includes various species, but it is larger than Dallister.
- **Spirulina:** This is a type of cyanobacteria and is also larger than Dallister.

4. **Conclusion:** Based on the analysis, Dallister is confirmed as the smallest bacterium among the options provided.

5. **Final Answer:** The smallest bacterium is **Dallister**.

---