Which organism primarily uses binary fission for reproduction?

Binary fission is a straightforward method of asexual reproduction primarily utilized by bacteria. In this process, a single bacterial cell replicates its genetic material and then divides into two identical daughter cells. This method allows for rapid population growth under favorable conditions, as each division can occur within a short time frame, sometimes as quickly as every 20 minutes.

Bacteria are unicellular organisms that lack the complex reproductive strategies found in multicellular organisms such as fungi, which typically reproduce through spores or budding. Viruses, on the other hand, do not reproduce independently but require a host cell to replicate. Protozoa are more complex single-celled organisms that often reproduce through methods such as mitosis or binary fission, but they also can employ other means including sexual reproduction.

This distinction illustrates how binary fission is predominantly associated with bacteria, making it the correct choice for the question posed.