We have to know that a represor protein is a molecule that inhibits RNA to transcribe a DNA fragment to eventually form amino acids of proteins. So a system in which lac operon genes are about to be transcribed is the one shown in diagram A, since we can see the RNA polymerase attached to the DNA fragment, instead of the represor. At this point, between the two options regarding the A diagram, we could say that the correct answer is "Diagram A because the repressor protein is no longer attached to the DNA segment".
