The correct answer is D) waterside deposits. Waterside deposits on boiler tubes can lead to the development of heat blisters. These deposits act as an insulator, preventing proper heat transfer from the flame to the water. This causes localized overheating of the tube material, leading to the formation of heat blisters. The other options are incorrect because: A) flame impingement would cause general tube thinning, not blistering; B) gas laning refers to uneven gas flow, which does not directly cause blistering; and C) insufficient water circulation would lead to overall tube overheating, not localized blistering.