The best answer is A.
A restriction enzyme is an enzyme that catalyzes the cleavage of DNA after recognizing a specific sequence of DNA.
Restriction enzymes are proteins produced by bacteria that cut DNA at specific sites. These enzymes are used by bacteria to defend against bacterial viruses known as bacteriophages. When a phage infects a bacteria, it inserts its DNA into the bacteria so as to be replicated.
The restriction enzyme thwarts the replication of the phage DNA by cutting it into many pieces. These enzymes have been isolated from bacteria by scientists and used as indispensable tools in recombinant DNA technology.