Answer; The correct answer for the blank is-
C) rough ER ( rough endoplasmic reticulum).
mRNA is translated to form protein at ribosomes.
The proteins that are required to be secreted outside the cell are first transported to the endomembrane system ( which includes endoplasmic reticulum and golgi complex).
In the endoplasmic reticulum, post translational modification of protein takes place ( such as proper folding of protein). They are dispatched from the ER and move to golgi complex.
Finally, the vesicles ( containing proteins) exit from golgi, fuse with the cell membrane and are released outside the cell.
Example– When insulin is synthesized, it passes through the ER, where it is cut, folded and held in proper shape with the help of disulfide (-S-S-) bridges.
Thus, option C) is the right answer.