The correct answer is A) regulate the superheater outlet temperature by cooling a portion of the superheated steam. The primary purpose of a control desuperheater installed in the steam drum of a boiler is to regulate the temperature of the superheated steam exiting the superheater. By cooling a portion of the superheated steam, the desuperheater helps maintain the desired temperature at the superheater outlet, which is crucial for the efficient and safe operation of the boiler system. The other options are incorrect because they do not accurately describe the primary purpose of the control desuperheater. Option B is incorrect as the desuperheater does not regulate the saturated steam temperature. Option C is incorrect as the desuperheater does not add moisture to the superheated steam. Option D is incorrect as the desuperheater's primary purpose is not to assure a constant volume of steam flow through the superheater.