Which microorganism is the largest in size?

The correct choice is protozoan, as these microorganisms typically exhibit a larger size compared to the other options provided. Protozoa are single-celled eukaryotic organisms and can range in size from about 10 to over 100 micrometers, depending on the species. This size range allows them to be viewed under a light microscope, making them larger than both bacteria and viruses.

In comparison, bacteria are generally much smaller, usually ranging from 0.5 to 5 micrometers, while viruses are even tinier, often between 20 and 300 nanometers in size. Algae, though they can vary significantly in size, often exist as multicellular organisms or larger single-celled forms that can reach considerable sizes, but many microscopic algae remain smaller than protozoa. Thus, protozoa possess the largest size among these microorganisms, making this the accurate answer.