The correct answer is A) operating at low steaming rates. In a regenerative air heater, air is bypassed around the heater when the boiler is operating at low steaming rates. This is done to maintain the desired air temperature entering the boiler, as the regenerative air heater may not be able to effectively heat the air at low flow rates. Bypassing the air around the heater ensures that the proper air temperature is maintained for efficient combustion, even when the boiler is running at a lower load. The other options are incorrect because they do not directly relate to the function of a regenerative air heater during low steaming rates. Option B is about the forced draft fans, option C is about surface blows, and option D is about blowing the tubes, none of which are specifically relevant to the air bypass in a regenerative air heater.