**Core Concept**
The rough endoplasmic reticulum (RER) is a type of organelle found in eukaryotic cells, characterized by the presence of ribosomes attached to its surface. It plays a crucial role in protein synthesis, folding, and modification, as well as lipid synthesis.

**Why the Correct Answer is Right**
The RER is involved in several key functions, including protein synthesis, where it serves as a site for translation of mRNA into protein. The attached ribosomes read the sequence of codons on the mRNA and assemble the corresponding amino acids into a polypeptide chain. The RER is also responsible for the folding and modification of proteins, including the addition of carbohydrates, lipids, and other molecules. Additionally, it is involved in lipid synthesis, producing various types of lipids such as phospholipids and cholesterol.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because protein synthesis is a primary function of the RER.
**Option B:** Incorrect because lipid synthesis is also a function of the RER.
**Option C:** Incorrect because the correct answer should not be listed.

**Clinical Pearl / High-Yield Fact**
The RER is often referred to as the "protein factory" of the cell, highlighting its critical role in protein synthesis and modification.

**Correct Answer:** C.