**Core Concept**
Post-transcriptional modification of RNA refers to the series of processes that occur after the transcription of DNA into RNA, but before the RNA is translated into a protein. These modifications are crucial for the stability, localization, and translation efficiency of the RNA molecule. Key modifications include capping, polyadenylation, and splicing.

**Why the Correct Answer is Right**
Glycosylation is a post-translational modification that occurs after the protein has been synthesized, not as a post-transcriptional modification of RNA. In glycosylation, a carbohydrate molecule is attached to the protein, which can affect its stability, folding, and function. This process is unrelated to RNA modification and occurs after translation.

**Why Each Wrong Option is Incorrect**
* **Option A:** 5' Capping is a post-transcriptional modification that protects the RNA from degradation and facilitates its translation by binding to initiation factors.
* **Option B:** Addition of a poly A tail is another post-transcriptional modification that stabilizes the RNA and facilitates its export from the nucleus.
* **Option C:** Removal of introns is part of the splicing process, which is a post-transcriptional modification that removes non-coding regions (introns) from the RNA and joins the coding regions (exons) together.

**Clinical Pearl / High-Yield Fact**
Key post-transcriptional modifications of RNA include 5' capping, polyadenylation, splicing, and microRNA processing, which all play critical roles in regulating gene expression and protein synthesis.

**✓ Correct Answer: D. Glycosylation is a post-translational modification, not a post-transcriptional modification of RNA.**