Which of the following organelles is not bound by a membrane?

Explanation

Ribosomes are the only organelles listed that do not have a membrane surrounding them; they are made up of ribosomal RNA and proteins, playing a vital role in translating mRNA into proteins. In contrast, the endoplasmic reticulum (ER), mitochondria, and nucleus are all membrane-bound organelles. The ER, consisting of rough and smooth regions, is essential for protein and lipid synthesis. Mitochondria, known for their double membranes, are responsible for energy production through ATP synthesis, while the nucleus, also double membrane-bound, contains the genetic material and coordinates cellular activities. Therefore, ribosomes stand out as the correct answer.