**Core Concept**
Methylation of cytosine residues is a crucial epigenetic modification that plays a significant role in gene regulation and expression. This process involves the addition of a methyl group to the 5th carbon atom of the cytosine ring, typically at CpG dinucleotides, leading to changes in chromatin structure and gene activity.

**Why the Correct Answer is Right**
Methylation of cytosine residues is catalyzed by the enzyme DNA methyltransferase (DNMT), which transfers a methyl group from S-adenosylmethionine (SAM) to the cytosine residue. This modification can silence gene expression by preventing transcription factors from binding to the promoter region or by recruiting repressive chromatin remodeling complexes. The resulting dense, methylated chromatin structure can also limit access to transcriptional machinery, thereby inhibiting gene expression.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because methylation of cytosine residues does not directly result in gene duplication. Gene duplication is a process that involves the creation of additional copies of a gene, often through errors during DNA replication or recombination.

**Option B:** This option is incorrect because methylation of cytosine residues does not directly result in the formation of a gene silencer. Gene silencers are regulatory elements that repress gene expression, but they are distinct from methylation of cytosine residues.

**Option C:** This option is incorrect because methylation of cytosine residues does not directly result in the activation of a gene. Methylation typically leads to gene silencing, although there are some exceptions where it can activate gene expression.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that methylation of cytosine residues is a reversible process, meaning that demethylation can occur through the action of enzymes like TET and TDG. This dynamic regulation of DNA methylation is crucial for maintaining proper gene expression and cellular function.

**Correct Answer: D. Hydroxymethylation**