Which of the following best describes a virus?

• A. An acellular infectious agent that requires a host cell to replicate
• B. A prokaryotic cell that reproduces by binary fission
• C. A unicellular organism capable of independent metabolism
• D. A multicellular parasite with its own metabolic machinery

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the definition of a virus. A virus is a microscopic infectious agent that is acellular, meaning it is not made up of cells.
2. Step 2: Recognize that viruses cannot carry out metabolic processes or reproduce independently; they require a host cell to replicate.
3. Step 3: Compare the options given: prokaryotic cells reproduce by binary fission and have cellular structures; unicellular organisms can metabolize independently; multicellular parasites have their own metabolism.
4. Step 4: Identify that the only option describing an acellular infectious agent dependent on a host cell for replication matches the characteristics of a virus.
5. Step 5: Conclude that the best description of a virus is 'An acellular infectious agent that requires a host cell to replicate.'