Bacteroids are

Options

A. Enlarged non-motile cellular bacteria Rhizobium leguminosarum in root nodules of legumes
B. A bacterial cell infected with viruses
C. A motile bacterium
D. Nitrosomonas bacteria in soil

Explanation

### Step-by-Step Solution:

1. **Definition of Bacteroids**: Bacteroids are a specific form of bacteria that are found in the root nodules of leguminous plants. They are derived from the genus Rhizobium, which is known for its role in nitrogen fixation.

**Hint**: Remember that Bacteroids are associated with leguminous plants and play a crucial role in nitrogen fixation.

2. **Cellular Characteristics**: Bacteroids are described as enlarged, non-motile cells. This means that unlike some bacteria, they do not have the ability to move on their own.

**Hint**: Focus on the size and motility of Bacteroids compared to other bacteria.

3. **Gram Staining**: Bacteroids are classified as Gram-negative bacteria. This classification is based on their cell wall structure, which has a thin layer of peptidoglycan and an outer lipid membrane.

**Hint**: Recall the significance of Gram-negative bacteria and their cell wall composition.

4. **Shape and Structure**: Bacteroids are typically rod-shaped, which is a common shape for many types of bacteria.

**Hint**: Think about the common shapes of bacteria when considering Bacteroids.

5. **Reproductive Characteristics**: Bacteroids do not form spores, specifically endospores, which are a survival mechanism for some bacteria under unfavorable conditions.

**Hint**: Understand the difference between spore-forming bacteria and those like Bacteroids that do not form spores.

6. **Oxygen Requirement**: Bacteroids are obligate anaerobes, meaning they cannot survive in the presence of oxygen. They require an oxygen-free environment to produce ATP and carry out their metabolic processes.

**Hint**: Remember the term "obligate anaerobe" and its implications for the habitat of Bacteroids.

7. **Infection and Viruses**: The term "bacteriophage" refers to viruses that infect bacteria, but this is not directly related to Bacteroids themselves. It is mentioned to clarify the context of bacterial infections.

**Hint**: Distinguish between Bacteroids and bacteriophages, as they serve different roles in biology.

### Summary:
Bacteroids are enlarged, non-motile, rod-shaped, Gram-negative bacteria found in the root nodules of legumes. They are obligate anaerobes that do not form spores and are crucial for nitrogen fixation in plants.