The correct answer is: Proteins that will be secreted from the cell are likely to be found in closed spaces bounded by membranes of the endomembrane system.
The endomembrane system is a group of organelles and their surrounding membranes in eukaryotic cells (prokaryotic cell doesn’t have membrane enveloped organelles) that works together in order to modify, package, and transport lipids and proteins. It includes nucleus with its nuclear envelope, lysosomes, the endoplasmic reticulum (smooth and rough), Golgi apparatus but also plasma membrane around the whole cell. The endomembrane system does not include mitochondria, chloroplasts, or peroxisomes.