The correct answer is D) all of the above. Excessive vibration in an auxiliary boiler can be caused by several factors: 1) Loose hold-down bolts - This can allow the boiler to move and vibrate excessively. 2) Insufficient air to the burner - This can lead to uneven combustion and combustion pulses, which can induce vibration. 3) Combustion pulses - Uneven or pulsing combustion in the boiler can directly cause vibration. All of these factors can contribute to excessive vibration in an auxiliary boiler, so D) all of the above is the correct answer.