The correct answer is A) mechanical tube cleaning. Waterside abrasion of boiler tubes can be caused by mechanical tube cleaning methods, such as the use of tube scrapers or other abrasive cleaning tools. This physical contact between the cleaning device and the tube walls can wear down and damage the tube material over time. The other answer choices are incorrect because: B) oxygen corrosion would affect the exterior of the tubes, not the waterside; C) improper bends in the tubes would not directly cause waterside abrasion; and D) impurities in the boiler water, while potentially problematic, do not directly lead to waterside abrasion of the tubes.