Members of which of the following groups are all parasites?

Explanation

Option A is correct. All viruses are obligate intracellular parasites, meaning they can only replicate inside living host cells because they lack metabolism and ribosomes. In contrast, not all fungi or algae are parasitic-some species are free-living, while others may cause disease. Bacteria are also not strictly parasites; they are independent single-celled organisms that can live freely or in association with hosts. Thus, viruses are the best example of true obligate parasites.