Final answer:
A 50 micrometer organism is most likely single-celled and eukaryotic, as this size exceeds the typical range for a prokaryotic cell.
Step-by-step explanation:
If you have isolated an organism that is 50 micrometers in size, this organism is most likely D) Single-celled. Prokaryotic cells, which consist of organisms from the domains Bacteria and Archaea, are usually much smaller, ranging from 0.1 to 5.0 micrometers in diameter. Since the organism in question is 50 micrometers, it exceeds the typical size range for prokaryotes. Most eukaryotic cells, which include animal and plant cells, have a size range that can accommodate the given measurement. Eukaryotic cells are characterized by having a nucleus and other membrane-bound organelles, which are not found in prokaryotic cells. Given that the average size of eukaryotic cells falls into the larger range, it's likely that the organism is eukaryotic and single-celled, such as an amoeba or a large protist.