Rna polymerase is able to open the dna double helix as it moves down the template. what type of enzymatic activity does this mean rna polymerase must possess?

This is the Helicase activity. 
RNA polymerase catalyzes the chemical reaction the synthesis RNA, called transcription, using gene's DNA template. The enzyme must search the DNA template for promoters which are sites on the DNA where it binds to start the process. The enzyme has intrinsic helicase activity, which means there is no separate enzyme that will be needed to Unwind the DNA, unlike in the case of DNA polymerase.

This post might have the info you need: