Which of the following plays an important role in nitrogen cycle?

Options

A. Yeast
B. Nitrobacter
C. Mucor
D. Spirogyra

Explanation

To determine which organism plays an important role in the nitrogen cycle from the given options (Yeast, Nitrobacter, Mucor, and Spirogyra), we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Nitrogen Cycle**: The nitrogen cycle involves several processes that convert nitrogen from one form to another in the environment. Key processes include nitrogen fixation, nitrification, and denitrification.

2. **Identify the Organisms**: We have four organisms to consider:
- Yeast: A type of fungus.
- Nitrobacter: A type of bacteria.
- Mucor: Another type of fungus.
- Spirogyra: A type of algae.

3. **Role of Each Organism**:
- **Yeast**: Primarily involved in fermentation; does not play a significant role in the nitrogen cycle.
- **Nitrobacter**: A bacteria that is specifically involved in the nitrification process, converting ammonia into nitrites and then into nitrates.
- **Mucor**: Another fungus, similar to yeast, and does not play a significant role in the nitrogen cycle.
- **Spirogyra**: An algae that does not directly contribute to the nitrogen cycle processes.

4. **Determine the Key Player**: Among the organisms listed, Nitrobacter is the only one that is a bacterium involved in the nitrification process, which is crucial for the nitrogen cycle.

5. **Conclusion**: Therefore, the organism that plays an important role in the nitrogen cycle is **Nitrobacter**.

### Final Answer:
**Nitrobacter** plays an important role in the nitrogen cycle.