**Core Concept**
The nucleosome is a fundamental structure in eukaryotic chromatin, consisting of a segment of DNA wrapped around a core of histone proteins. This structure is essential for the organization and regulation of genetic material.

**Why the Correct Answer is Right**
The correct answer, C. Regular repeating structure of DNA & histone proteins, is accurate because nucleosomes are characterized by their repeating pattern of DNA wrapped around a core of histone proteins. This structure, known as the nucleosome repeat unit (NRU), typically consists of 147 base pairs of DNA wrapped around a core of eight histone proteins (two copies each of histones H2A, H2B, H3, and H4). The regular repeating structure of nucleosomes is crucial for the packaging and regulation of eukaryotic DNA.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because nucleosomes consist of two copies each of four different histone proteins (H2A, H2B, H3, and H4), not just one type of histone protein.

**Option B:** This option is incorrect because non-histone proteins, such as linker histones and transcription factors, play a role in chromatin organization and regulation, but they do not separate individual nucleosomes from each other.

**Option D:** This option is incorrect because the term "small nucleus" is not a relevant description of the nucleosome. The nucleus is a larger organelle that contains the chromatin, whereas the nucleosome is a smaller structural unit within the chromatin.

**Clinical Pearl / High-Yield Fact**
A key aspect of nucleosome structure is the linker DNA, which connects adjacent nucleosomes and is susceptible to epigenetic modifications that regulate gene expression. Understanding the nucleosome structure is crucial for appreciating the mechanisms of gene regulation and epigenetic modifications.

**✓ Correct Answer: C. Regular repeating structure of DNA & histone proteins**