The correct answer is A) Poor heat transfer due to hard scale deposits overheats tubes. Hard scale deposits on the inside of boiler tubes restrict the efficient transfer of heat from the boiler's heat source to the water within the tubes. This reduced heat transfer can cause the metal of the tubes to overheat, potentially leading to tube failure and boiler damage. The other answer choices, while related to boiler operations, do not directly address the primary issue of poor heat transfer caused by hard scale deposits.