What is the main characteristic that distinguishes rough endoplasmic reticulum from smooth endoplasmic reticulum?

The rough endoplasmic reticulum (RER) is primarily distinguished from the smooth endoplasmic reticulum (SER) by the presence of ribosomes attached to its cytoplasmic surface. These ribosomes give the RER its "rough" appearance under a microscope. The primary function of the RER is the synthesis of proteins that are either secreted from the cell, incorporated into the cell's plasma membrane, or sent to an organelle. In contrast, the smooth endoplasmic reticulum lacks ribosomes and plays a different role in the cell, such as lipid synthesis, detoxification processes, and calcium ion storage.

Thus, the presence of ribosomes on the RER is the key characteristic that differentiates it from the smooth endoplasmic reticulum, making this the correct choice.