**Core Concept**
The Golgi apparatus is a complex organelle found in eukaryotic cells responsible for protein modification, sorting, and packaging. It plays a crucial role in the post-translational modification of proteins, including glycosylation, phosphorylation, and proteolytic processing.

**Why the Correct Answer is Right**
The enzyme marker for the Golgi apparatus is **N-acetylglucosaminyltransferase (GnT)**, specifically the enzyme involved in the transfer of N-acetylglucosamine to the terminal mannose residue of a growing carbohydrate chain. This process is a key step in the glycosylation of proteins, which is a characteristic feature of the Golgi apparatus. The correct enzyme is **UDP-N-acetylglucosamine:** 2-acetamido-2-deoxy-D-glucose N-acetylglucosaminyltransferase.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the Golgi apparatus or protein modification.
* **Option B:** This option is incorrect as it is related to the endoplasmic reticulum, not the Golgi apparatus.
* **Option C:** This option is incorrect as it is related to protein synthesis, not protein modification.

**Clinical Pearl / High-Yield Fact**
The Golgi apparatus is responsible for the glycosylation of proteins, which is essential for their proper function and stability. Defects in glycosylation can lead to various diseases, including lysosomal storage disorders and congenital disorders of glycosylation.

**Correct Answer: D. UDP-N-acetylglucosamine:** 2-acetamido-2-deoxy-D-glucose N-acetylglucosaminyltransferase