The function of restriction endonucleases
(a) Useful in genetic engineering
(b) Protects the bacterial DNA against foreign DNA
(c) Both a and b
(d) Helpful in photosynthesis

Hint: Restrictions endonucleases cut the two strands of DNA at different locations which are used to form recombinant DNA molecules by using various techniques and it also masks the bacterial DNA by adding methyl groups.

Complete answer:
In Escherichia coli, the growth of bacteriophage is restricted by two enzymes. They both were isolated. In those two enzymes, one enzyme adds a methyl group to DNA, and the other enzyme cuts the DNA. The enzymes that cut the DNA are called restriction endonucleases. Their function depends on a specific DNA nucleotide sequence which was isolated and characterized later. To form recombinant molecules of DNA, restriction endonucleases are used in genetic engineering. They also help in protecting the bacterial DNA from foreign DNA by masking their own DNA with methyl groups.

Additional information:
- Nucleases are the larger class of enzymes that contain restriction enzymes. They are of two types namely exonuclease and endonucleases.
- The function of restriction endonucleases depends on the isolated specific DNA nucleotide sequence which was characterized later
- Nucleotides at the end of the DNA are removed by exonucleases and cuts at specific positions within the DNA are done by restriction endonucleases. They function by inception the length of the DNA sequence.
- Molecules of DNA that are composed of DNA from different sources or genomes are formed by using restriction endonucleases in genetic engineering.

So, the correct answer is ‘both a and b’.

Note:
-Each restriction endonuclease can detect a specific palindromic sequence of nucleotides in the DNA.
-Most common restriction endonucleases make the staggered cut which means cutting the two strands of DNA at different locations.