Restriction endonucleases recognise specific sequences in the DNA and cut it into small fragments. The specific sites are 4-6 bp long and are called as restriction sites. Each restriction endonuclease has a specific restriction site. Ligase enzyme joins two large molecules by forming a new chemical bond; for example, DNA ligase joins two DNA fragments together via phosphodiester bond between two nucleotides.
Exonucleases enzymes remove nucleotides one at a time from the end of a polynucleotide chain. They shorten the DNA by one nucleotide at a time, they do not digest it into fragments.
DNA denaturation by helicases creates topological stress in the helical DNA structure which is relieved by the action of topoisomerases. These enzymes transiently break both DNA strands of one chromosome thereby making the other chromosome to pass through the break and relieve the stress.
EcoRI is present in E. coli as a part of the restriction modification system. The nucleic acid sequence of its restriction site is 5' GAATTC 3'. It cuts the DNA between G and A in the above-mentioned sequence. Since EcoRI does not cut the DNA randomly but has specific restriction site and cut DNA within the nucleotide chain; it is a restriction endonuclease.
So, the correct answer is option A.