Answer:
The correct answer is - they observed ribosomes on the structure.
Explanation:
The endoplasmic reticulum is the cellular organ eukaryotic organisms, that is tubular in shape and present in the cytoplasm that plays role in protein folding and part of the transport system of the cell. It has two subunits smooth endoplasmic reticulum and rough endoplasmic reticulum.
Rough endoplasmic reticulum has ribosome molecules present on the surface of the structure that is sending the protein to ER where folding of the protein and transport of the protein.