Which of the following is a type of membranous organelle?

The endoplasmic reticulum is a type of membranous organelle because it consists of a network of membranous tubules and sacs that are interconnected. This organelle plays a crucial role in the synthesis of proteins and lipids, as well as in the processing and transporting of these molecules within the cell. Its structure includes both rough and smooth regions; the rough endoplasmic reticulum is studded with ribosomes, which are essential for protein synthesis, while the smooth endoplasmic reticulum is involved in lipid synthesis and detoxification processes.

In contrast, the cytoskeleton, while crucial for maintaining cell shape and enabling cellular movement, is composed of protein filaments and does not have a membrane structure. Ribosomes are complexes made of RNA and proteins that are responsible for protein synthesis, but they are not considered membranous organelles because they lack a surrounding membrane. Cilia are hair-like extensions on the surface of some cells, important for movement and sensing the environment, but they also do not fit the description of a membranous organelle. Thus, the endoplasmic reticulum stands out as the correct answer in the context of membranous organelles.