When a piano tuner hears a beat frequency between a tuning fork and a piano wire, it indicates that the frequencies are close but not the same. To reduce the beat frequency, the tuner should adjust the tension in the wire to make its frequency match that of the tuning fork more closely.
Therefore, the correct response is to decrease the amount of interference and decrease the beat frequency. This means the tuner should increase or decrease the tension in the wire to align the frequencies better, reducing the beat frequency and the interference caused by the dissonance between the two sounds.