Final answer:
option D. Ribosomes. The rough ER is involved in protein synthesis due to the ribosomes attached to its surface, which translate mRNA into proteins.
Step-by-step explanation:
Protein synthesis can occur on the rough endoplasmic reticulum (ER) but not on the smooth ER due to the presence of ribosomes attached to the surface of the rough ER. These ribosomes are the organelles responsible for assembling proteins from individual amino acids by translating mRNA - a process known as translation. The lack of ribosomes gives the smooth ER a smooth appearance and a different set of functions, such as lipid synthesis and drug detoxification, which do not involve protein synthesis.