**Core Concept**
Eukaryotic DNA contains various regions that contribute to protein synthesis. Exons are the coding regions that ultimately form the polypeptide chain, while enhancers are regulatory sequences that increase the transcription of a gene.

**Why the Correct Answer is Right**
The correct answer involves two key components of eukaryotic DNA: exons and enhancers. Exons (option a) are the coding regions of a gene that contain the sequence information necessary for the synthesis of a polypeptide chain. Enhancers (option b) are regulatory DNA sequences that can increase the transcription of a gene by binding to specific transcription factors. This binding can either enhance or inhibit the transcription of a gene. The correct answer, D. ab, indicates that both exons and enhancers contribute to polypeptide synthesis.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because leader sequences (option c) are not directly involved in polypeptide synthesis. Leader sequences are found at the 5' end of a gene and can contain regulatory elements, but they do not code for the polypeptide chain.
**Option B:** Incorrect because tRNA (option d) is not a component of eukaryotic DNA. tRNA (transfer RNA) molecules are involved in protein synthesis, but they are transcribed from their own genes and are not part of the eukaryotic DNA sequence.
**Option C:** Incorrect because leader sequences (option c) are not directly involved in polypeptide synthesis, as explained above.
**Option E:** Incorrect because ncRNA (option e) refers to non-coding RNA, which can include various types of RNA molecules that do not code for proteins. While some ncRNA molecules can be involved in regulating gene expression, they are not directly involved in polypeptide synthesis.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that exons are the coding regions of a gene, and enhancers can significantly influence gene expression by increasing transcription. This understanding is crucial for interpreting genetic data and understanding the regulation of gene expression in eukaryotic cells.

**✓ Correct Answer: D. ab. Both exons and enhancers contribute to polypeptide synthesis.**