**Core Concept**
RNA processing involves a series of modifications that occur after transcription, including splicing, capping, and polyadenylation. These processes ensure that the mature RNA molecule is suitable for translation or other cellular functions. The modifications can be divided into two main categories: addition and modification of nucleosides.

**Why the Correct Answer is Right**
Chemical hydrolysis does not occur during the processing of RNA. Instead, the correct processes involve the addition of nucleosides (terminal addition) and their modification (nucleoside modification). These modifications are essential for the proper functioning of the RNA molecule. For example, the addition of a 5' cap and a poly-A tail during polyadenylation helps to protect the RNA molecule from degradation and facilitates its export from the nucleus.

**Why Each Wrong Option is Incorrect**
**Option A:** Terminal addition is a correct process that occurs during RNA processing, where nucleotides are added to the 3' end of the RNA molecule.
**Option B:** Nucleoside modification is also a correct process that involves the addition of chemical groups to the nucleosides, which is essential for the proper functioning of the RNA molecule.
**Option C:** Nucleoside cleavage does not occur during the processing of RNA, but rather during the breakdown of RNA molecules, such as during RNA degradation.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that RNA processing involves the addition and modification of nucleosides, but not the breakdown of nucleosides. This distinction is crucial for understanding the mechanisms of RNA biology and its relevance to various diseases.

**Correct Answer Line**
✓ Correct Answer: D. Chemical hydrolysis