**Core Concept**
Supercoiling is a process where DNA becomes twisted upon itself, resulting in a helical structure. This phenomenon is particularly significant in prokaryotic cells, where the absence of histone proteins leads to a more compact and supercoiled DNA conformation.

**Why the Correct Answer is Right**
Supercoiling occurs due to the unwinding of DNA during replication and transcription. As the double helix is unwound, the lagging strand is left with a negative superhelical tension, causing the DNA to twist upon itself. This process is facilitated by topoisomerase enzymes, which introduce or remove supercoils from the DNA molecule. The supercoiling of DNA is essential for maintaining its stability and facilitating the processes of transcription and replication.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because supercoiling is not a characteristic of eukaryotic chromatin, where DNA is wrapped around histone proteins to form a more compact structure.
* **Option B:** This option is incorrect because DNA denaturation refers to the unwinding of the double helix, but it does not involve the twisting of DNA upon itself.
* **Option C:** This option is incorrect because the formation of nucleosomes is a process that occurs in eukaryotic cells, where DNA is wrapped around histone proteins, but it is not directly related to supercoiling.

**Clinical Pearl / High-Yield Fact**
Topoisomerase inhibitors, such as etoposide, are used in cancer chemotherapy to target rapidly dividing cells by preventing DNA replication and transcription.

**Correct Answer: D. DNA in prokaryotic cells.**