Final answer:
The key difference between bacteria and protists is that bacteria are prokaryotes lacking a nucleus and membrane-bound organelles, while protists are eukaryotes with a defined nucleus and the potential for various organelles.
Step-by-step explanation:
The difference between protists and bacteria revolves around their cellular organization. The correct answer to the question 'Which answer best explains the difference between protists and bacteria?' is A. Bacteria are prokaryotes and protists are eukaryotes.
Bacteria, which include both Eubacteria and Cyanobacteria, are characterized by their simple cellular structure without a nucleus or other membrane-bound organelles. They do, however, have ribosomes for protein synthesis and a cell membrane and wall surrounding the cell. On the other hand, protists, which are likely the first eukaryotic organisms to evolve, possess a more complex cellular structure with a defined nucleus and the capability to have various other organelles like mitochondria and chloroplasts.
Therefore, option B, C, and D are incorrect because protists also have a cell membrane, ribosomes, and cytoplasm just like bacteria. The main distinction lies in the presence of a nucleus and membrane-bound organelles in protists, whereas bacteria lack these features.