Cyanobacteria are classified under

Options

A. protista
B. plantae
C. monera
D. algae.

Explanation

### Step-by-Step Solution:

1. **Understanding Cyanobacteria**:
- Cyanobacteria are often referred to as blue-green algae due to their color and ability to perform photosynthesis. However, they are not true algae.

2. **Classification Overview**:
- Organisms are classified into different kingdoms based on their cellular structure and characteristics. The major kingdoms include Monera, Protista, Fungi, Plantae, and Animalia.

3. **Identifying the Characteristics of Cyanobacteria**:
- Cyanobacteria are prokaryotic organisms, meaning they lack a defined nucleus and other membrane-bound organelles. This places them in the kingdom Monera, which consists of prokaryotic organisms.

4. **Distinguishing Between the Options**:
- **Protista**: This kingdom includes eukaryotic organisms, which are not applicable to cyanobacteria.
- **Plantae**: This kingdom consists of multicellular eukaryotic plants, which also do not include cyanobacteria.
- **Monera**: This kingdom includes prokaryotic organisms, making it the correct classification for cyanobacteria.
- **Alga**: While cyanobacteria are often called blue-green algae, they are not classified as true algae and are instead categorized under Monera.

5. **Conclusion**:
- Based on the characteristics and classification of cyanobacteria, the correct answer is that they are classified under the kingdom **Monera**.

### Final Answer:
Cyanobacteria are classified under **Monera**.