The most abundant microorganism are: -

The correct option is B

Bacteria

The correct option is B.

Explanation of the correct option: -

1. Bacteria are also the most common biological forms.
2. They can be found in the air, on land, in water, and even inside animals and plants' bodies.
3. Some bacteria are harmful, while others are beneficial.
4. Bacteria help the human digestive system by recycling nutrients in the soil.

Explanation of the incorrect options:-

Option A: -

A virus is a submicroscopic infectious agent that replicates only within an organism's live cells. Viruses infect all living things, including bacteria and archaea, from mammals and plants to microbes.
Option C: -

1. Paramecium is a genus of protozoans that are tiny, single-celled, and free-living.
2. The majority of species are easily cultivated in the laboratory, making them suitable model organisms for biological research.
3. Paramecium range in size from 0.05 to 0.32 mm in length (0.002 to 0.013 inch).

Option D:-

1. Amoeba is a single-celled organism with the ability to change its shape.
2. Ponds, lakes, and slow-moving rivers are common places for them to live.
3. These single-celled organisms can also infect humans and cause a wide range of diseases.

Hence most abundant microorganisms are bacteria.