How Do Positive Regulators Stimulate Initiation of Transcription by RNA

How do positive regulators stimulate initiation of transcription by RNA polymerase?

A) by allowing passage of the polymerase through the operator
B) by binding to the polymerase and increasing its ability to initiate transcription
C) by causing the helix to unwind in the operator allowing easier initiation
D) by making the transcription start site more exposed to the polymerase