Rhizobium bacterium is a common example of which of the following groups of bacteria?

Options

A. Nitrifying
B. Ammonifying
C. Nitrogen fixing
D. Denitrifying

Explanation

To determine which group of bacteria the Rhizobium bacterium belongs to, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Role of Rhizobium**:
- Rhizobium is known for its ability to fix nitrogen. It forms a symbiotic relationship with leguminous plants, allowing it to convert atmospheric nitrogen into a form that plants can use.

2. **Identify the Groups of Bacteria**:
- The question provides four options:
1. Nitrifying bacteria
2. Ammonifying bacteria
3. Nitrogen-fixing bacteria
4. Denitrifying bacteria

3. **Define Each Group**:
- **Nitrifying Bacteria**: These bacteria convert ammonia into nitrites and then into nitrates (a process called nitrification).
- **Ammonifying Bacteria**: These bacteria convert organic nitrogen from dead organisms into ammonia (a process called ammonification).
- **Nitrogen-fixing Bacteria**: These bacteria convert atmospheric nitrogen into ammonia, making it available for plants. Rhizobium falls into this category.
- **Denitrifying Bacteria**: These bacteria convert nitrates back into nitrogen gas, releasing it into the atmosphere (a process called denitrification).

4. **Analyze the Options**:
- Since Rhizobium is known for its nitrogen-fixing ability and establishes a symbiotic relationship with legumes, it clearly fits into the category of nitrogen-fixing bacteria.

5. **Select the Correct Answer**:
- The correct answer is **Nitrogen-fixing bacteria**.

### Final Answer:
Rhizobium bacterium is a common example of **Nitrogen-fixing bacteria**.