**Core Concept**
Splicing is a fundamental process in molecular biology that involves the removal of introns and the joining of exons in a pre-mRNA molecule to form a mature mRNA molecule. This process is crucial for the accurate expression of genes and is mediated by a complex machinery of enzymes and small nuclear ribonucleoproteins (snRNPs).

**Why the Correct Answer is Right**
During splicing, the spliceosome, a large complex of snRNPs and associated proteins, recognizes and removes introns from the pre-mRNA molecule. The spliceosome then joins the exons together using a process known as ligation, which is catalyzed by the enzyme RNA ligase. This results in the formation of a mature mRNA molecule that can be translated into protein.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe the process of splicing. While transcription is a related process, it involves the synthesis of pre-mRNA from a DNA template, whereas splicing involves the modification of pre-mRNA.
* **Option B:** This option is incorrect because it describes a process known as editing, which involves the modification of individual nucleotides in a pre-mRNA molecule.
* **Option C:** This option is incorrect because it describes a process known as translation, which involves the synthesis of protein from a mature mRNA molecule.

**Clinical Pearl / High-Yield Fact**
Splicing defects can lead to a range of diseases, including muscular dystrophy, cystic fibrosis, and spinal muscular atrophy. These diseases are often caused by mutations in genes that encode for proteins involved in the splicing process, highlighting the importance of accurate splicing for gene expression.

**Correct Answer: D. Splicing is a process of removing introns and joining exons in a pre-mRNA molecule to form a mature mRNA molecule.**