Although restriction enzymes are quite efficient at digestion of foreign DNA, they are prevented from acting on the DNA of host cells by: The inability of host cell DNA to be bound to the active site of the enzyme. The presence of competitive inhibitors which prevent the enzymes from binding to the host DNA. The methylation of bases in recognition sequences in host cell DNA. The inaccessibility of host cell DNA for the enzymes.

Restriction enzymes are quite efficient at digestion of foreign DNA, they are prevented from acting on the DNA of host cells by the methylation of bases in recognition sequences in host cell DNA - Option C.

Explanation:

A bacterial cell that produces a restriction enzyme does not have its own DNA, although , due to restriction modification system, it contains diverse restriction sites for the recognition enzyme.

The modification system includes the modification enzyme - DNA methylase, which recognizes the modification site and modifies the site by the addition of a methyl group to one or two bases.