The correct answer is D. The main purpose of the bias resistor Rb in the illustrated circuit (figure "A") is to eliminate the need for an emitter-base battery for establishing forward emitter-base bias. The bias resistor Rb provides a path for the base current, allowing the transistor to operate in the active region by establishing the necessary forward emitter-base bias without the need for an additional emitter-base battery. The other options are incorrect because they do not accurately describe the purpose of the bias resistor Rb in this circuit. Option A and B are incorrect as the bias resistor is not used to establish reverse biases. Option C is incorrect as the bias resistor is used to establish forward, not reverse, collector-emitter bias.