Rough endoplasmic reticulum (RER) are responsible for the assembly of proteins. A signal peptide is a short peptide that is present at the N-terminus of the newly synthesized proteins. It is cleaved by signal peptidase in RER and then enters the lumen of rough endoplasmic reticulum. Protein folding takes place in the lumen of the endoplasmic reticulum. Smooth endoplasmic reticulum (SER) is involved in the synthesis of phospholipid and cholesterol. Protein glycosylation takes place in the lumen of the RER. Thus, the correct answer is option C.