Blue green algae is also referred to as cyanobacteria
cyanobacteria is in a group of unicellular prokaryotes,in Monera
cyanobacteria can prepare it's own food

The correct answer is option 'A' - Cyanobacteria.

Cyanobacteria, commonly known as blue-green algae, are a group of photosynthetic bacteria that obtain energy through photosynthesis. They are prokaryotic organisms, meaning they lack a true nucleus and other membrane-bound organelles. Cyanobacteria are found in various environments including freshwater, marine, and terrestrial habitats. They have the ability to carry out oxygenic photosynthesis, similar to plants, which involves using sunlight to convert carbon dioxide and water into glucose and oxygen.

Below are the reasons why the correct answer is option 'A' - Cyanobacteria:

Cyanobacteria are photosynthetic bacteria:
- Cyanobacteria have pigments such as chlorophyll a, which enables them to carry out photosynthesis. This is a characteristic shared with plants, algae, and other photosynthetic organisms.
- Through photosynthesis, cyanobacteria produce glucose as a source of energy and release oxygen as a byproduct.

Cyanobacteria are prokaryotes:
- Prokaryotes are organisms whose cells lack a membrane-bound nucleus and other organelles. Cyanobacteria belong to this group.
- They have a simple cellular structure and lack a true nucleus. The genetic material is present in the form of a single circular DNA molecule.

Cyanobacteria are widespread in various habitats:
- Cyanobacteria are found in a wide range of environments, including freshwater bodies like lakes and ponds, marine environments, and even in symbiotic associations with other organisms.
- They can tolerate extreme conditions such as high temperatures, high salinity, and low nutrient availability, allowing them to colonize diverse habitats.

Cyanobacteria are often referred to as blue-green algae:
- The term "blue-green algae" is a common name given to cyanobacteria due to their ability to carry out photosynthesis and their appearance, which can range from blue-green to green.
- However, it is important to note that cyanobacteria are prokaryotes (bacteria), not true algae, which are eukaryotic organisms.

In conclusion, the blue-green algae are correctly referred to as cyanobacteria. They are photosynthetic bacteria that lack a true nucleus and are found in various environments. The term "blue-green algae" is used due to their ability to carry out photosynthesis, but it is important to recognize that they are prokaryotic organisms.