Final answer:
Viruses are non-cellular entities that depend on host cells to replicate, which poses challenges in studying them due to their need for live cellular environments and the complexity introduced by their vast genetic diversity.
Step-by-step explanation:
Viruses are defined as non-cellular entities which establish themselves as obligate intracellular parasites. They are acellular, which means they do not have cellular structures such as organelles and ribosomes, and lack the ability to metabolize or grow on their own. Instead, they depend on a host cell's machinery to replicate, by directing the host to produce new viral components. The correct answer to the question is A) Non-cellular entities; Genetic diversity.
Studying viruses in a lab setting is challenging because of their dependency on host cells for replication and their incredible genetic diversity. Viruses can only be seen with electron microscopes and their evolutionary origin is still largely a matter of speculation due to the lack of historical evidence like fossils. Their non-cellular nature and the requirement of a host cell for reproduction make them unique study subjects compared to cellular organisms.