Before replication can take place. (1) A. DNA polymerases must add complementary nucleotides to DNA. B. the two strands of DNA must separate. C. the covalent bonds in DNA must break. D. helicases must break the bonds in the nucleotides.

The correct answer is B. the two strands of DNA must separate.

Explanation:

DNA is a double-stranded structure. The two strands are joined together by the formation of hydrogen bonds between complementary nucleotides. So before the replication takes place these two strands must be separated.

The separation of DNA strand is accomplished by the action of a helicase enzyme which breaks the hydrogen bond between complementary nucleotides and separated the two DNA strands.

After separation, the DNA polymerase binds to separated DNA and replicating the DNA. So the right answer is B.