Salutations!
If you observed a cell under a microscope and saw that it contained a plasma membrane, cell wall, and ribosomes, but no other organelles, what could you conclude?
If you observed a cell under a microscope and saw that it contained a plasma membrane, cell wall, and ribosomes, but no other organelles. I'd conclude that it is a prokaryotic cell. Prokaryotic cells are often called "before life" cells. They are basic and ancient, and are 200 times smaller than Eukaryotes. They do not contain a nucleus,nor cell bound membrane, but of the advantage is that they can reproduce quite easily.
Hope I helped (:
Have a great day!