Answer:
A. 
prokaryotic cell
Explanation:
The cells of living organisms have been classified as either prokaryotic or eukaryotic depending on whether or not they have a membrane-bound nucleus. Prokaryotic cell are cells that do not possess a membrane-bound nucleus while eukaryotic cells possess a membrane-bound nucleus that houses their genetic material.
According to this question, a student observes a cell through a microscope and notices that it does not have a nucleus but it does have a cell wall, a cell membrane, and numerous ribosomes. Due to the fact that this cell does not have a NUCLEUS, irrespective of any other organelle it has, it is considered a PROKARYOTIC CELL.
