Which type of endoplasmic reticulum contains ribosomes?

The rough endoplasmic reticulum (RER) is distinguished by the presence of ribosomes attached to its cytoplasmic surface. These ribosomes are the sites of protein synthesis, where mRNA is translated into polypeptide chains. The RER plays a crucial role in the synthesis of proteins that are either secreted from the cell, incorporated into the cell's plasma membrane, or sent to an organelle such as the lysosome.

In contrast, the smooth endoplasmic reticulum (SER) lacks ribosomes and is primarily involved in the synthesis of lipids, metabolism of carbohydrates, and detoxification of drugs and poisons. Since the question specifically asks which type of endoplasmic reticulum contains ribosomes, the rough endoplasmic reticulum is clearly the correct response, as its function is directly related to the attachment of these ribosomes for the synthesis of proteins.