What type of cell lacks a membrane-enclosed nucleus?

The correct answer is prokaryotic cell because these cells are characterized by the absence of a membrane-enclosed nucleus. Instead of having a defined nucleus that houses their genetic material, prokaryotic cells, which include bacteria and archaea, have their DNA located in a nucleoid region that is not separated from the rest of the cell by a membrane.

In contrast, eukaryotic cells possess a distinct nucleus that is enclosed by a nuclear membrane where the genetic material is organized within chromatin. This structural difference is fundamental, as it categorizes cells into two major groups: prokaryotes and eukaryotes. The lack of a membrane-bound nucleus in prokaryotic cells is one of the key distinctions that define their simpler structure and functioning compared to the more complex organization of eukaryotic cells.

Both eukaryotic and prokaryotic cells are critical to the diversity of life, but the defining characteristic of a prokaryotic cell is its lack of a membrane-enclosed nucleus. Therefore, prokaryotic cells clearly fit the description given in the question.