What describes the main purpose of rna polymerase during transcription?

The enzyme, RNA polymerase, which performs the transcription process, binds to the promoter sequence and then beings to work its way down the DNA segment, constructing RNA to match the DNA nucleotides over which the enzyme passes.

RNA polymerase produces mRNA from DNA

this is true, assuming that the allele is completely recessive and incomplete dominance is not a factor.