To distinguish between bacteria and protists, the presence or absence of certain cellular structures is crucial. Given the options you have provided, the trait least likely to help you identify the correct group of these organisms is:
D: the absence of cell structure membranes.
Both bacteria (prokaryotes) and protists (eukaryotes) have cell membranes surrounding their cells, though bacterial cells usually do not have membrane-bound organelles. The presence of cell structures (like a nucleus) and various cell shapes can help differentiate between these groups more effectively.