The normal function for restriction endonuclease is to 

Restriction endonucleases recognise specific sequences in the DNA and digest the DNA into fragments by destroying the phosphodiester bonds. Hence, the restriction endonucleases are synthesised by bacteria as part of their defense mechanism which help in the cleavage of the DNA segments which are identified as foreign. The restriction sites are very short sequences and can be present in the genome of the bacteria. Its own DNA is protected from destruction by adding methyl groups to it. Certain bases in the strand of DNA of the bacteria are modified for getting protected against the activity of the enzymes.

Thus, the correct option is C.