Question

Restriction enzymes were first discovered with the observation that

Answer 1

phage DNA is destroyed in a host cell 
c. 
Now let know more about it:-
Restriction enzymes were first discovered in the 1950s when evidence for a sort of primitive immune system was found to exist in bacteria. It was observed that certain strains of E. coli 
were resistant to infection by various bacteriophages. This phenomenon seemed to be a property of the bacterial cell itself, an ability to restrict the growth and replication of the attacking phages. In 1962, at the University of Geneva, it was found that the resistant bacterium possessed an enzyme system that selectively recognized and destroyed foreign phage DNA within the bacterial membrane and at the same time modified the chromosomal DNA of the bacterium 
to prevent self-destruction.