The correct answer is D. All of the above.
Viruses are not regarded as cells because they lack many of the characteristics that define cells. They are not able to replicate on their own, and they do not have the ability to carry out all of the basic functions that are necessary for life, such as metabolism and respiration.
Viruses are essentially just a piece of genetic material (DNA or RNA) that is surrounded by a protein coat. They are not able to function on their own, and they must infect a host cell in order to replicate. Once inside a host cell, the virus takes over the cell’s machinery and uses it to produce more copies of itself. These new viruses then burst out of the host cell and infect other cells.
Because viruses do not have all of the characteristics that define cells, they are not considered to be living things. However, they are still able to cause disease in humans and other animals.