What type of transport can further sort proteins in the ER?

Vesicular transport is essential for further sorting proteins in the endoplasmic reticulum (ER). This process involves the formation of vesicles that bud off from membranes, allowing proteins to be encapsulated and transported to specific destinations within or outside the cell. In the ER, as proteins are synthesized and processed, they are packaged into transport vesicles that can then move to the Golgi apparatus or other cellular locations, ensuring that they reach their appropriate sites for further modification and function.

While active transport involves the movement of substances against their concentration gradient and is critical in various cellular functions, it does not specifically address the sorting process within the ER. Facilitated diffusion pertains to the passive transport of molecules through channel proteins, which is also not relevant for the sorting of proteins in the ER. Passive transport is the movement of substances along their concentration gradient without the use of energy, but again does not specifically pertain to the sorting task that is carried out by vesicular transport in the ER. Thus, vesicular transport is the most accurate answer regarding the mechanism used for sorting proteins in this cellular compartment.