Bacteria can be seen only

The correct option is C

under a microscope

The correct option is (C):
Explanation for the correct option:

1. Bacteria are the single-celled microbes which are heterotrophic in nature.
2. Bacteria are organisms which are not visible by naked eyes.
3. Bacteria can only be visualized under electron microscope as it is able to magnify objects by 300 times.
4. Bacteria can be both useful for making food products like idli, curd or can also be harmful and cause diseases.
5. Bacteria differ in shapes- spherical, curved etc.
6. Examples- Salmonella, Pseudomonas

Final answer: Bacteria can be seen only under a microscope.