The correct answer is C) dirty atomizers. If a boiler is smoking black and increasing the boiler front air box pressure does not reduce the smoke, the most likely cause is dirty atomizers. Atomizers are responsible for properly atomizing and spraying the fuel into the boiler. When the atomizers become dirty, they cannot effectively atomize the fuel, leading to incomplete combustion and black smoke. Adjusting the air box pressure would not resolve this issue, as the problem is with the fuel delivery system rather than the air supply. The other options are incorrect because: A) a forced draft fan failure would affect the air supply, which can be addressed by adjusting the air box pressure; B) high ambient air temperature would not cause the specific issue of black smoke and unresponsiveness to air box pressure adjustment; and D) heavy soot on the tubes would not directly impact the fuel atomization process.