Which of the following is the largest single membrane-bound intracellular compartment?

The endoplasmic reticulum is the biggest organelle in most eukaryotic cells, surrounded by a continuous membrane. Its membrane accounts for approximately 50% of all cell membranes, while the space enclosed by the ER accounts for around 10% of total cell volume. In certain locations, the Endoplasmic reticulum membrane resembles a single membrane (often three-layered); in others, it may have a micellar (globular) form. The ER is a network of flattened, membrane-bound structures, and its lumen is the confined space between physiologically active ER membranes. 

Therefore, ER is the largest structure in the cell that is surrounded by a single continuous membrane.