The correct answer is B) waterside scale deposits. Blisters on boiler tubes are typically caused by waterside scale deposits. As mineral salts and other impurities accumulate on the inside of the boiler tubes, they can create an insulating layer that prevents efficient heat transfer. This leads to localized overheating of the tube material, which in turn causes the metal to blister and potentially rupture. The other options are incorrect because: A) hot feed water would not directly cause blisters, C) cold feed water may contribute to scale formation but is not the primary cause, and D) air in the feed water may lead to other boiler issues but is not a direct cause of blistering.