**Core Concept**
Nucleosomes are the fundamental units of chromatin structure, formed during the process of DNA packaging in eukaryotic cells. They consist of a segment of DNA wound around a core of histone proteins.

**Why the Correct Answer is Right**
The correct answer is **A. Histone** because nucleosomes are formed when a segment of DNA, approximately 147 base pairs in length, wraps around a core of eight histone proteins. These histones, specifically histones H2A, H2B, H3, and H4, are the primary protein components of the nucleosome. The histone core provides a scaffold for the DNA to wrap around, facilitating the compaction of chromatin into higher-order structures.

**Why Each Wrong Option is Incorrect**
**Option B:** DNA is not the primary component of the nucleosome; rather, it is the DNA that wraps around the histone core.
**Option C:** RNA is not a component of nucleosomes; RNA is involved in transcription and translation processes, not chromatin structure.
**Option D:** DNA & RNA both are not correct because RNA is not a component of nucleosomes.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that nucleosomes play a crucial role in regulating gene expression by influencing the accessibility of DNA to transcription factors and other regulatory proteins. This is achieved through post-translational modifications of histones, such as acetylation and methylation, which can either relax or compact chromatin structure.

**✓ Correct Answer: A. Histone**