Answer: topoisomerase prevents formation of positive supercoil.
Explanation:
During replication,at the replication fork, the helicases functions to unwind the DNA. As unwinding occurs, the DNA upstream becomes overwound/positively supercoiled and become tangled, just like experimenting with a length of rope. So, for the cell to avoid DNA tangling, the topoisomerases helps in reducing/preventing overwounding by forming breaks in the phosphate backbone of the DNA so that they rotae and become relaxed and then the topoisomerase seals up the breaks. The topoisomerases are differentiated by the number of breaks they make.