Which organelle carries substances like proteins to various parts of the cell?

The Endoplasmic Reticulum (ER) is the correct answer because it serves as the primary site for the synthesis and transport of proteins and lipids within the cell. It is a network of membranes that can be classified into two types: rough ER, which is studded with ribosomes and primarily responsible for the synthesis of proteins that will be secreted or incorporated into membranes, and smooth ER, which synthesizes lipids and detoxifies certain chemicals.

The Golgi Apparatus also plays a critical role in processing and packaging proteins for transport but is more focused on modifying, sorting, and delivering proteins that have already been synthesized in the ER. Mitochondria are involved in energy production, while chloroplasts perform photosynthesis in plant cells. Thus, while the Golgi Apparatus is important for processing, the ER is the organelle responsible for the initial transportation of proteins within the cell.