DNA is unzipped by RNA polymerase. I'm unsure as to why your textbook says it is unzipped by helicase, since DNA helicase is involved in DNA replication. RNA polymerase isn't biochemically a helicase, even though it does partly function as one. However, that said, I do think the biology behind transcription and the specific enzymes is much more complicated but that's beyond VCE. Just stick to RNA polymerase and clarify this with your biology teacher.
Also, it's important to remember that with transcription, a transcription factor is needed to bind to the promoter region in order for RNA polymerase to attach to the DNA and begin the transcription process.
