Explanation:

Introduction:
DNA methylation is a process in which methyl groups are added to the DNA molecule. This process is carried out by enzymes called DNA methyltransferases. Methylation of DNA plays a crucial role in various biological processes, including gene expression, genomic imprinting, and DNA repair.

Protecting DNA from Restriction Enzymes:
Restriction enzymes are enzymes that recognize specific DNA sequences and cleave the DNA at those sites. Bacteria have restriction enzymes as part of their defense mechanism against foreign DNA, such as viruses. These enzymes can also potentially cleave the bacterial DNA itself if not protected. Methylation of DNA is one way bacteria can protect their own DNA from being cleaved by their own restriction enzymes. When a bacterium adds methyl groups to its DNA, it modifies the DNA sequence and prevents the restriction enzymes from recognizing and cleaving the DNA. This protects the bacterium's own DNA from being disrupted.

Gene Expression:
Gene expression refers to the process by which information from a gene is used to synthesize a functional gene product, such as a protein. Methylation of DNA can play a role in gene expression by turning genes on or off. Methylation of specific regions of DNA, known as CpG islands, can silence or inhibit the expression of genes. When methyl groups are added to the DNA, it can prevent the transcription machinery from accessing the gene and transcribing it into RNA. This effectively turns the gene off and prevents its expression.

Conclusion:
In conclusion, a bacterium modifies its DNA by adding methyl groups to protect its DNA from its own restriction enzymes. Methylation of DNA prevents the restriction enzymes from recognizing and cleaving the DNA. Additionally, methylation can also play a role in gene expression by turning genes on or off. Methylation of specific regions of DNA can silence or inhibit the expression of genes. Hence, option D is the correct answer.