The enzyme that facilitates opening of the DNA helix during transcription is RNA polymerase.

RNA polymerase is responsible for the synthesis of RNA from a DNA template during transcription. It binds to the DNA at a specific site called the promoter region and then unwinds the DNA helix to expose the template strand. This opening of the DNA helix is facilitated by the activity of RNA polymerase, which breaks the hydrogen bonds between the base pairs of the DNA.

Once the DNA is unwound, RNA polymerase can begin the synthesis of RNA by adding complementary RNA nucleotides to the template strand. The RNA polymerase moves along the DNA strand, opening the helix as it goes, and synthesizing a new RNA molecule.

Therefore, option B, "RNA polymerase," is the correct answer.