The correct answer is C) buildup of alternator voltage. When there is an open occurring within the field rheostat of an AC generator, short-circuiting the terminals will result in the buildup of alternator voltage. This is because with the field rheostat open, the field current is cut off, causing the voltage regulator to attempt to increase the field current by driving the field to a higher voltage. This increased voltage can be observed as the alternator voltage builds up. The other options are incorrect because a negative or positive deflection of the wattmeter pointer (A and B) would indicate a short circuit, not an open circuit. Additionally, a low but constant alternator voltage (D) would not be expected with an open circuit in the field rheostat.