Which of the following is required by viruses in order to reproduce?

• A. A host cell
• B. Nutrient-rich agar plates
• C. Direct sunlight
• D. Free-living in the environment

Answer: (A)

Verified step by step guidance

1. Understand that viruses are obligate intracellular parasites, meaning they cannot reproduce on their own and must infect a living host cell to replicate.
2. Recognize that unlike bacteria or other microorganisms, viruses lack the cellular machinery (such as ribosomes) necessary for protein synthesis and energy production.
3. Identify that nutrient-rich agar plates and direct sunlight are not sufficient for viral reproduction because viruses do not metabolize nutrients or perform photosynthesis.
4. Note that free-living in the environment does not support viral reproduction since viruses require the metabolic processes of a host cell to replicate their genetic material and assemble new virus particles.
5. Conclude that the essential requirement for viral reproduction is a host cell, which provides the necessary machinery and environment for the virus to replicate.