Final answer:
The endoplasmic reticulum, especially the rough endoplasmic reticulum with attached ribosomes, is the cell organelle involved in protein synthesis.
Step-by-step explanation:
The cell organelle involved in protein synthesis is the endoplasmic reticulum, specifically the rough endoplasmic reticulum, where ribosomes are attached. The process starts with the synthesis of the protein on the ribosome, followed by modification in the Golgi apparatus, packaging in the endoplasmic reticulum, and sometimes further modification in transport vesicles. Therefore, the correct answer is C) Endoplasmic reticulum.
Protein synthesis starts in the ribosomes, where mRNA is translated into proteins, and then the proteins are modified and packaged in the endoplasmic reticulum. The Golgi apparatus is involved in modifying and packaging proteins, but it is not directly involved in protein synthesis.