Final answer:
The organelle not involved in protein synthesis is the Smooth ER, as its functions include lipid synthesis and detoxification, but not protein assembly. The option (C) is correct.
Step-by-step explanation:
The organelle not involved in protein synthesis is Smooth ER. Protein synthesis is a critical cellular process, and while the ribosome is the actual site where proteins are assembled, the Rough ER is involved as it is studded with ribosomes on its surface.
The nucleus also plays a role in protein synthesis as it contains the DNA which is used to make mRNA through the process of transcription. However, the Smooth ER has functions unrelated to protein synthesis, such as lipid synthesis, metabolism of carbohydrates, and detoxification of drugs and poisons. Therefore, option (C) is correct.