Final answer:
Many proteins are glycosylated in the endoplasmic reticulum, specifically the rough endoplasmic reticulum (RER), as part of the protein synthesis and processing that occurs within the endomembrane system.
Step-by-step explanation:
The main answer to the question 'Many proteins are glycosylated in which location?' is C. the endoplasmic reticulum. Glycosylation is the process where sugars are covalently attached to proteins, and it commences in the rough endoplasmic reticulum (RER). The RER is a pivotal part of the endomembrane system, which includes the plasma membrane, nuclear envelope, lysosomes, vesicles, endoplasmic reticulum, and Golgi apparatus. The endomembrane system works collectively to modify, tag, package, and transport proteins and lipids. After synthesis and initial glycosylation in the RER, proteins are then transported to the Golgi apparatus for further modification and packaging into vesicles for their correct destinations, which may include extracellular fluids, lysosomes, or other organelles.