Final answer:
If an organism is single-celled, the only conclusion that can be drawn with certainty is that it is microscopic. While it could be a prokaryote, which is a single-celled organism without a nucleus, not all single-celled organisms are prokaryotes; some may be single-celled eukaryotes.
Step-by-step explanation:
Understanding Single-Celled Organisms
If an organism is single-celled, it can be inferred that it is not a complex multicellular organism. This means option D is incorrect. Regarding option A, all organisms, including single-celled organisms, possess genetic material, so the conclusion that it lacks genetic material is also incorrect. Option C suggests it is likely a prokaryote; this could be a plausible conclusion as prokaryotes are unicellular organisms that lack a nucleus, but not all single-celled organisms are prokaryotes, many are eukaryotes like protists. Hence, option B, which states that it is microscopic, is the most accurate since single-celled organisms are generally too small to be seen without the aid of a microscope.
A prokaryote is a type of single-celled organism that lacks a nucleus and other membrane-bound organelles, with its DNA located in a region called the nucleoid. Prokaryotes include organisms within the domains Bacteria and Archaea. On the other hand, a microorganism could be a single-celled eukaryote or prokaryote. It is only when the cell lacks organelles, such as in the domains Bacteria and Archaea, that we can confidently conclude the cell is prokaryotic. Therefore, without further information, we cannot definitively say the single-celled organism is a prokaryote, but we can confirm that it is microscopic in size.