**Core Concept**
In eukaryotic gene expression, the process of removing non-coding regions from the primary transcript to generate mature mRNA is crucial for producing a functional protein. This phenomenon is a result of the splicing process, which involves the precise removal of introns and the joining of exons to form a continuous coding sequence.

**Why the Correct Answer is Right**
The correct answer refers to the non-coding regions of a eukaryotic gene that are removed during the splicing process. These regions, known as introns, do not contribute to the final protein sequence and are therefore not represented in the mature mRNA. The splicing process involves the recognition of specific sequences at the exon-intron junctions by the spliceosome, a complex of small nuclear ribonucleoproteins (snRNPs). The spliceosome catalyzes the removal of the intron and the joining of the adjacent exons, resulting in a mature mRNA molecule that can be translated into a protein.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specifically refer to the non-coding regions of a eukaryotic gene that are removed during splicing.
* **Option B:** This option is incorrect because it is a type of RNA that is involved in protein synthesis, but it does not refer to the non-coding regions of a eukaryotic gene.
* **Option D:** This option is incorrect because it refers to a type of genetic variation that involves the insertion or deletion of nucleotides, but it does not specifically refer to the non-coding regions of a eukaryotic gene.

**Clinical Pearl / High-Yield Fact**
In eukaryotic gene expression, the splicing process is crucial for producing a functional protein. Mutations in the splice sites or in the genes encoding the snRNPs can lead to aberrant splicing and the production of aberrant mRNAs, which can result in a variety of diseases, including muscular dystrophy and spinal muscular atrophy.

**Correct Answer:** C. intron