**Core Concept**
Splicing is a critical process in RNA processing, where introns (non-coding regions) are removed, and exons (coding regions) are joined together to form a mature mRNA molecule. This process is essential for the production of functional proteins from the primary transcript of a gene.

**Why the Correct Answer is Right**
Small nuclear RNA (snRNA) molecules, particularly those involved in the spliceosome complex, have splicing activity. These snRNAs, such as U1, U2, U4, U5, and U6, are crucial for recognizing and removing introns, and joining exons together. The spliceosome complex, comprising these snRNAs and other proteins, catalyzes the splicing reaction, allowing for the efficient removal of introns and the formation of a mature mRNA molecule.

**Why Each Wrong Option is Incorrect**
**Option A:** mRNA (messenger RNA) is the product of splicing, not the molecule with splicing activity. It carries the genetic information from DNA to the ribosome for protein synthesis.
**Option B:** miRNA (microRNA) is a type of small non-coding RNA that regulates gene expression by binding to mRNA and preventing its translation. It does not have splicing activity.
**Option C:** rRNA (ribosomal RNA) is a structural component of ribosomes, where protein synthesis occurs. It is not involved in splicing.

**Clinical Pearl / High-Yield Fact**
The spliceosome complex is a fascinating example of RNA-protein interactions, where snRNAs and proteins work together to catalyze the splicing reaction. Understanding the splicing process is essential for appreciating the complexity of gene expression and the regulation of protein synthesis.

**✓ Correct Answer: D. sn RNA. Small nuclear RNA.**