## Core Concept
Histone modifications play a crucial role in regulating gene expression by altering chromatin structure. These modifications can occur through various post-translational mechanisms, including acetylation, methylation, phosphorylation, ubiquitination, and SUMOylation. These modifications can either relax or compact chromatin structure, thereby influencing gene expression.

## Why the Correct Answer is Right
Histone acetylation is a well-known post-translational modification that involves the addition of an acetyl group to the lysine residues on the histone tails. This process is catalyzed by histone acetyltransferases (HATs) and can lead to a more open chromatin structure, facilitating gene transcription.

## Why Each Wrong Option is Incorrect
* **Option A:** Histone methylation is a post-translational modification that involves the addition of one, two, or three methyl groups to lysine or arginine residues on histones. This can either activate or repress gene transcription, depending on the specific histone and degree of methylation.
* **Option B:** Histone phosphorylation is another post-translational modification that typically occurs on serine, threonine, or tyrosine residues. This modification is often associated with DNA damage response and can influence chromatin structure and gene expression.
* **Option D:** Histone ubiquitination involves the addition of ubiquitin proteins to histones, which can have various effects on gene expression and DNA damage response.

## Clinical Pearl / High-Yield Fact
A key point to remember is that these histone modifications are dynamic and can be reversible, allowing for fine-tuned regulation of gene expression. Aberrant histone modifications have been implicated in various diseases, including cancer.

## Correct Answer: D.