**Core Concept**
The mapping of hypersensitive sites in recombinant DNA research involves identifying specific regions of DNA that are more accessible to enzymes or other proteins. This process is crucial for understanding gene regulation and chromatin structure.

**Why the Correct Answer is Right**
DNase I is an enzyme that selectively cleaves DNA in regions where the chromatin is more accessible, such as near transcription factor binding sites or other regulatory elements. This results in a pattern of DNA fragments that can be used to map hypersensitive sites. DNase I is often used in conjunction with techniques like Southern blotting or next-generation sequencing to identify these regions.

**Why Each Wrong Option is Incorrect**
**Option A:** DNA ligase is an enzyme that seals gaps between DNA fragments, often used in cloning and PCR reactions, but it does not cleave DNA to map hypersensitive sites.
**Option B:** DNA polymerase I is an enzyme that synthesizes new DNA strands, often used in PCR reactions, but it does not cleave DNA or map hypersensitive sites.
**Option D:** Polynucleotide kinase is an enzyme that adds phosphate groups to DNA, often used in labeling DNA fragments, but it does not cleave DNA or map hypersensitive sites.

**Clinical Pearl / High-Yield Fact**
When working with DNA, it's essential to understand the specific functions of different enzymes to avoid contamination or incorrect results. DNase I is a critical tool in molecular biology for mapping hypersensitive sites, but it requires careful handling and control to produce accurate results.

**✓ Correct Answer: C. DNase I**