**Core Concept**
DNA methylation is a crucial epigenetic mechanism that plays a vital role in regulating gene expression by modifying the DNA molecule. It involves the addition of a methyl group to the 5th carbon atom of the cytosine residue, typically in a CpG dinucleotide context, which can lead to gene silencing or repression.

**Why the Correct Answer is Right**
DNA methylation is catalyzed by DNA methyltransferases (DNMTs), specifically DNMT1, which maintains methylation patterns during DNA replication, and DNMT3A and DNMT3B, which introduce new methylation marks during development and cell differentiation. The methylation of CpG islands in gene promoter regions can lead to the recruitment of transcriptional repressors and the silencing of gene expression. This epigenetic modification is crucial for embryonic development, X-chromosome inactivation, and the regulation of cellular differentiation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because DNA methylation primarily involves the addition of a methyl group to cytosine, not guanine.
**Option B:** This option is incorrect because DNA demethylation involves the removal of methyl groups from the DNA molecule, which is a distinct process from DNA methylation.
**Option C:** This option is incorrect because histone modification, while also an epigenetic mechanism, involves the alteration of histone proteins surrounding the DNA molecule, rather than the direct modification of the DNA molecule itself.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that DNA methylation is a reversible process, and changes in methylation patterns can be influenced by environmental factors, such as diet and exposure to toxins.

**Correct Answer:** D.