The correct answer is D) regulate the superheater outlet temperature by cooling a portion of the superheated steam. A control desuperheater installed in the steam drum of a boiler is primarily used to regulate the temperature of the superheated steam exiting the superheater. By injecting a small amount of cooler saturated steam into the superheated steam, the desuperheater can lower the overall temperature, ensuring that the steam entering the turbine or other equipment is at the desired temperature. The other options are incorrect because they do not accurately describe the primary purpose of a control desuperheater. Option A is incorrect as the desuperheater does not add moisture to regulate temperature, but rather injects a small amount of cooler steam. Option B is incorrect as the desuperheater is not used to assure a constant volume of steam flow. Option C is incorrect as the desuperheater regulates the superheated steam temperature, not the saturated steam temperature.