**Core Concept**
DNA methylation is a type of epigenetic modification that involves the addition of a methyl group to the cytosine residue in a CpG dinucleotide, resulting in gene silencing. This process is crucial for regulating gene expression, imprinting, and cellular differentiation.

**Why the Correct Answer is Right**
DNA methylation is a reversible process that can be influenced by various factors, including environmental stimuli and cellular stress. The enzyme responsible for DNA methylation is DNA methyltransferase (DNMT), which adds a methyl group to the cytosine residue. DNA methylation plays a significant role in cancer development, as it can lead to the silencing of tumor suppressor genes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because DNA methylation is indeed a reversible process, but it's not the only mechanism involved in gene expression regulation. Other epigenetic modifications, such as histone modifications and non-coding RNA-mediated regulation, also play crucial roles.
**Option B:** This option is incorrect because DNA methylation is not directly involved in the replication process. However, it does play a role in maintaining epigenetic marks during cell division.
**Option C:** This option is incorrect because DNA methylation is not directly associated with the development of autoimmune diseases. However, epigenetic dysregulation, including DNA methylation, has been implicated in the pathogenesis of autoimmune diseases.

**Clinical Pearl / High-Yield Fact**
DNA methylation is a crucial epigenetic modification that can influence gene expression in response to environmental stimuli, cellular stress, and disease states. Understanding the mechanisms of DNA methylation is essential for developing targeted therapies for various diseases, including cancer.

**Correct Answer: D.**