What enzyme unwinds the DNA double helix during replication?

Helicase is the enzyme that unwinds the DNA double helix during replication.

At a replication fork, helicase moves along the DNA and separates its two complementary strands by disrupting the interactions between paired bases. Many helicases use energy from ATP hydrolysis to drive this molecular-motor activity. The resulting single-stranded templates can then be copied by DNA polymerase.

The other enzymes in the choices have different jobs. Primase makes short RNA primers, polymerase adds DNA nucleotides to the growing strands, and ligase seals breaks between DNA fragments, especially on the lagging strand. Helicase activity is not limited to replication: related enzymes also separate nucleic-acid strands during transcription, repair, recombination, and other processes.

Unwinding creates torsional stress in the DNA ahead of the fork, so topoisomerases help relieve that strain. Single-strand-binding proteins then help keep the separated templates from immediately re-forming a double helix.