The correct answer is B) Small diameter tubes reduce gas turbulence in the tube banks. Smaller diameter boiler tubes experience less gas turbulence as the flue gases flow through the tube bank. This reduced turbulence results in more efficient heat transfer from the gases to the boiler water, which is the advantage of using smaller diameter tubes. The other answer choices are incorrect because: A) Smaller tubes do not reduce the overall heating surface area, C) Greater heat transfer rates are not an advantage of smaller tubes, and D) Tube diameter does not directly affect the insulating properties of soot buildup.