The correct answer is A) The diffuser is burned out. When the diffuser of a mechanically atomized burner is burned out, it can no longer properly atomize the fuel, causing the flame to be blown away from the burner tip. The diffuser is responsible for breaking up the fuel into a fine mist, allowing it to mix effectively with the air and ignite. If the diffuser is damaged or worn out, the fuel will not be properly atomized, leading to an unstable flame that can be easily blown away. The other options are incorrect because: B) Insufficient excess air would cause the flame to be smothered, not blown away; C) Low fuel viscosity would result in poor atomization, but not necessarily a blown-away flame; and D) Improper adjustment of the secondary air cone would affect the air-fuel mixture, but not necessarily cause the flame to be blown away.