Answer:
A: A system of folded membranes
Step-by-step explanation:
The endoplasmic reticulum is one of the organelles found in an eukaryotic cell. It is a membrane-bound organelle made up of two structures called CISTERNAE (sac) AND SHEET (membranous)
The endoplasmic reticulum is part of the endomembrane system, which is a system of folded membranes, which extends to the membrane that bounds the nucleus. It is of two types viz: rough endoplasmic reticulum and smooth endoplasmic reticulum.