Which one of the following types of infectious agents are single-celled organisms that can exist as spores?

Bacteria are indeed single-celled organisms, and some types of bacteria have the ability to form spores. This spore formation is a survival mechanism that allows them to withstand harsh environmental conditions, such as extreme heat, dehydration, or exposure to disinfectants. The process of sporulation enables bacteria to remain dormant for extended periods and can then reactivate to grow when conditions are favorable again.

In contrast, fungi can also exist as spores, but they are typically considered more complex organisms than bacteria because they can be multicellular (like molds) or unicellular (like yeasts). However, the specific query asks about single-celled organisms, which makes bacteria the more fitting answer in this context.

Prions, on the other hand, are not organisms but rather misfolded proteins that can induce other proteins to misfold, leading to disease. Viruses also do not classify as single-celled organisms; they are much smaller entities that require a host cell to replicate and do not possess cellular structures. Thus, the characteristics that define bacteria as single-celled organisms capable of spore formation rightly identify them as the correct answer to this question.