The correct answer is B) an open winding between the reference point "X" and the 25% tap connection. If 450 volts AC is measured across the load, this indicates that the autotransformer is not functioning properly. The issue is likely an open winding between the reference point "X" and the 25% tap connection, which would prevent the voltage from being stepped down as intended. This would result in the full source voltage of 450 VAC being applied to the load, rather than the expected lower voltage. The other answer choices are incorrect because: A) a properly operating circuit would not result in the full source voltage being applied to the load; C) an open winding between "Y" and the 75% tap would not account for the 450 VAC measured across the load; and D) a ground on one side of the supply voltage would not necessarily result in the full source voltage being measured across the load.