Final answer:
The largest known virus is the mimivirus, notable for being large enough to be visible under a light microscope and potentially playing a role in causing pneumonia in humans. The correct option is c.
Step-by-step explanation:
The largest known virus is the mimivirus, which is larger than some bacteria and is visible through a light microscope. Discovered in 1992 within an amoeba and officially identified as a virus in 2003, the mimivirus can be mistaken for a bacterium due to its large size.
Its diameter is about 400 nm, and it boasts protein filaments that extend from its surface, giving it an overall diameter of approximately 600 nm.
Most viruses are much smaller than the mimivirus, typically ranging from 20 nm to 900 nm, well below the threshold of visibility under a light microscope.
The mimivirus's size is an exception to the general rule, making it an interesting study for virologists. As for the mimivirus's potential role in human disease, it is believed that it may cause certain types of pneumonia.