Which of the following cellular structures lacks a membrane?

The nucleoid is the correct answer because it is the region within a prokaryotic cell where the genetic material (DNA) is located. Unlike eukaryotic cells, which have a defined nucleus surrounded by a nuclear membrane, the nucleoid does not have a surrounding membrane. Instead, it is simply a concentrated area of DNA in the cytoplasm of the cell.

In contrast, the other options all possess membranes. The nucleus, found in eukaryotic cells, is surrounded by a double membrane known as the nuclear envelope, which separates it from the cytoplasm. Ribosomes, while not membrane-bound themselves, are typically found either freely floating in the cytoplasm or attached to the endoplasmic reticulum, which is an extensive membranous network. The endoplasmic reticulum is a system of membranous tubules and sacs that plays a crucial role in the synthesis and transport of proteins and lipids, clearly indicating its membrane-bound nature. Thus, the presence or absence of a membrane clearly differentiates these cellular structures.