Which of the following is a common characteristic of microsporidia?

Microsporidia are indeed classified as obligate intracellular parasites, which means that they are unable to replicate outside of a host cell. This characteristic is crucial to understanding their biology and pathogenicity. As obligate intracellular parasites, they have evolved to invade and reside within the host's cells, where they exploit the host's cellular machinery for their own reproduction and survival. This property distinguishes them from many other types of parasites that may not have such a strict dependence on host cells for their life cycle.

The other options do not accurately describe microsporidia. They are not multicellular; instead, they are typically unicellular, and this follows the general classification of them as protists. Microsporidia also do not predominantly cause respiratory infections; their infections are more commonly associated with the intestinal tract or other systemic issues, especially in immunocompromised hosts. Finally, while there may be some associations with insect vectors, microsporidia are not predominantly transmitted by insects; they are mainly spread through spores in contaminated water or food, or through direct contact with infected hosts. Thus, the defining characteristic of being obligate intracellular parasites is what makes the correct answer stand out.