The answer is B: breaking hydrogen bonds. Nucleotide base pairs in the two complementary strands of DNA are linked to each other by weak hydrogen bonds which allows for the easy separation of the two strands. During replication, an enzyme known as helicase attaches itself to the unwound helix and begins to break the hydrogen bonds between the base pairs of the two DNA strands thereby pulling them apart to form a Y shape called a replication fork. This is what is called "unzipping". The relative weakness of the hydrogen bonds between the base pairs makes it fairly simple for helicase to break them apart and thus the task is accomplished very rapidly.