The correct answer is A, the "promoter" site is the RNA polymerase binding site on a typical gene. This is how the process of gene expression begins. The RNA polymerase (a protein that builds RNA) binds to the promoter site of the DNA strand. It then begins the process of 'transcribing' the DNA code into an RNA strand, which can then be 'translated' into an amino acid chain to form a protein, which is the expression of the gene.