Which structure is covered with ribosomes?

The rough endoplasmic reticulum (RER) is the correct choice because it is characterized by the presence of ribosomes on its cytoplasmic surface. These ribosomes give the RER a "rough" appearance, which distinguishes it from the smooth endoplasmic reticulum that lacks ribosomes. The primary function of the RER is to synthesize proteins that are either secreted from the cell, incorporated into the cell's plasma membrane, or sent to an organelle called the lysosome. The ribosomes attached to the RER translate messenger RNA (mRNA) into polypeptide chains, which then enter the lumen of the rough ER for further modifications and folding.

In contrast, structures like the smooth endoplasmic reticulum are involved in processes such as lipid synthesis and do not have ribosomes attached. The nuclear envelope, while it encases the nucleus, is also not covered in ribosomes. Lastly, the Golgi apparatus is involved in the modification and sorting of proteins but does not have ribosomes on its surface either. Understanding the roles of these cellular structures and their characteristics helps clarify the specific function of the rough endoplasmic reticulum in protein synthesis.