**Core Concept**
The question tests the understanding of **mRNA processing** in eukaryotic cells, specifically the removal of **introns** and the retention of **exons**. This process is crucial for generating mature messenger RNA (mRNA) from pre-mRNA transcripts.

**Why the Correct Answer is Right**
The correct answer is related to the fact that **introns** are non-coding regions that are removed during mRNA processing, while **exons** are coding regions that are retained. The sequence that is not expressed in mature mRNA would be one that is removed during this processing.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might represent an exon, which is retained in the mature mRNA.
**Option B:** Similarly, this could be another exon or a sequence that is part of the mature mRNA.
**Option C:** This option might also represent a sequence that is retained in the mature mRNA.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **introns** are removed from pre-mRNA during the splicing process, which is mediated by the **spliceosome**. This process is essential for producing mature, functional mRNA that can be translated into protein.

**Correct Answer:** D. Intron.