Protein transcription does occur in the ER. Specifically, the ribosomes are embedded into the ER. The ER should not be mistaken for the cytoplasm. It is a network of cell membrane that can be rough (many ribosomes for proteins synthesis) or smooth ER (which is not really involved in translation of proteins). Proteins are made and travel through the rough ER where they fold and maybe get other modifications. The proteins basically are modified with carbohydrate "tags" and lipid modifications in the golgi, which is almost an extension of the ER.
The key to this question (I over looked this too when I took the exam) is that you have to identify in the passage and know "retrograde transport." This protein was essentially made but was "tagged" to "serve it's purpose" in the ER membrane. It had to be transported back via a retrograde transport which is usually along actin filaments. Anterograde transport is away from the cell and retrograde is toward the cell. By the way - the ER does create vesicles to transport protein like the plasma membrane makes vesicles sometimes and the golgi. Usually these proteins from the ER go to either the cell membrane or golgi via vesicles. That supports the passage because this protein is responsible for vesicle formation.
