The answer is:
the change in red blood cell volume is principally due to the movement of WATER.
The ER synthesizes most substances of the cell, and then the Golgi body ins the one to classify, distribute, and modify them.
The proteins forming the cytoskeleton are the ones that hold everything in place.
The main proteins doing this are the microtubules, microfilaments, and Intermediate filaments.
Also, mention the carrier proteins, that move substances inside the cell.