**Core Concept**
Post-transcriptional modification of RNA is a crucial step in the synthesis of mature RNA molecules, where various chemical modifications are introduced to the primary transcript to enhance its stability, function, and localization. These modifications can occur in the nucleus or cytoplasm and involve the addition or removal of chemical groups, such as methyl, phosphate, or pseudouridine.

**Why the Correct Answer is Right**
Post-transcriptional modification of RNA involves several processes, including splicing, capping, polyadenylation, and editing. Splicing involves the removal of introns and joining of exons to form a mature mRNA molecule. Capping involves the addition of a methylated guanine nucleotide to the 5' end of the mRNA, while polyadenylation involves the addition of a poly-A tail to the 3' end. Editing involves the modification of specific nucleotides within the RNA sequence. However, one process that does not take place during post-transcriptional modification is the addition of a phosphate group to the RNA backbone.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because splicing is a crucial process that takes place during post-transcriptional modification, where introns are removed and exons are joined to form a mature mRNA molecule.
* **Option B:** This option is incorrect because capping is a well-established process that involves the addition of a methylated guanine nucleotide to the 5' end of the mRNA.
* **Option C:** This option is incorrect because polyadenylation is a critical process that involves the addition of a poly-A tail to the 3' end of the mRNA.
* **Option D:** This option is incorrect because editing involves the modification of specific nucleotides within the RNA sequence, which is a recognized process during post-transcriptional modification.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that post-transcriptional modification of RNA is a dynamic process that can occur in both the nucleus and cytoplasm, and involves a range of chemical modifications that can significantly impact the function and stability of the mature RNA molecule.

**Correct Answer: D.**